* lisp/vc/vc.el (vc-diff-mergebase, vc-log-mergebase): New commands.

* lisp/vc/vc-git.el (vc-git-mergebase): New function.
(vc-git-print-log): Interpret string value of arg LIMIT as an end-revision.

* lisp/vc/vc-hooks.el (vc-prefix-map): Bind 'vc-log-mergebase' to
'C-x v M L', and 'vc-diff-mergebase' to 'C-x v M D'.  (Bug#33950)

4 files changed, 75 insertions, 4 deletions

diff --git a/etc/NEWS b/etc/NEWS
index ad01bd8516b..afee1e1dcad 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -511,6 +511,10 @@ and compares their entire trees.
 *** New user option 'vc-hg-revert-switches' specifies switches to pass
 to hg revert.
 
+*** 'C-x v M D' ('vc-diff-mergebase') and 'C-x v M L' ('vc-log-mergebase')
+print diffs and logs between the merge base (common ancestor) of two
+given revisions.
+
 ** Diff mode
 +++
 *** Hunks are now automatically refined by font-lock.
diff --git a/lisp/vc/vc-git.el b/lisp/vc/vc-git.el
index 0f8c9c836ce..a921ff1bb88 100644
--- a/lisp/vc/vc-git.el
+++ b/lisp/vc/vc-git.el
@@ -101,6 +101,7 @@
 
 (eval-when-compile
   (require 'cl-lib)
+  (require 'subr-x) ; for string-trim-right
   (require 'vc)
   (require 'vc-dir))
 
@@ -1017,7 +1018,8 @@ This prompts for a branch to merge from."
 If SHORTLOG is non-nil, use a short format based on `vc-git-root-log-format'.
 \(This requires at least Git version 1.5.6, for the --graph option.)
 If START-REVISION is non-nil, it is the newest revision to show.
-If LIMIT is non-nil, show no more than this many entries."
+If LIMIT is a number, show no more than this many entries.
+If LIMIT is a revision string, use it as an end-revision."
   (let ((coding-system-for-read
          (or coding-system-for-read vc-git-log-output-coding-system)))
     ;; `vc-do-command' creates the buffer, but we need it before running
@@ -1045,8 +1047,14 @@ If LIMIT is non-nil, show no more than this many entries."
                     ,(format "--pretty=tformat:%s"
                              (car vc-git-root-log-format))
                     "--abbrev-commit"))
-                (when limit (list "-n" (format "%s" limit)))
-                (when start-revision (list start-revision))
+                (when (numberp limit)
+                  (list "-n" (format "%s" limit)))
+                (when start-revision
+                  (if (and limit (not (numberp limit)))
+                      (list (concat start-revision ".." (if (equal limit "")
+                                                            "HEAD"
+                                                          limit)))
+                    (list start-revision)))
                 '("--")))))))
 
 (defun vc-git-log-outgoing (buffer remote-location)
@@ -1077,6 +1085,10 @@ If LIMIT is non-nil, show no more than this many entries."
                         "@{upstream}"
                       remote-location))))
 
+(defun vc-git-mergebase (rev1 &optional rev2)
+  (unless rev2 (setq rev2 "HEAD"))
+  (string-trim-right (vc-git--run-command-string nil "merge-base" rev1 rev2)))
+
 (defvar log-view-message-re)
 (defvar log-view-file-re)
 (defvar log-view-font-lock-keywords)
@@ -1093,7 +1105,7 @@ If LIMIT is non-nil, show no more than this many entries."
            (cadr vc-git-root-log-format)
          "^commit *\\([0-9a-z]+\\)"))
   ;; Allow expanding short log entries.
-  (when (memq vc-log-view-type '(short log-outgoing log-incoming))
+  (when (memq vc-log-view-type '(short log-outgoing log-incoming mergebase))
     (setq truncate-lines t)
     (set (make-local-variable 'log-view-expanded-log-entry-function)
          'vc-git-expanded-log-entry))
diff --git a/lisp/vc/vc-hooks.el b/lisp/vc/vc-hooks.el
index 2052ace12bf..07b3d86b518 100644
--- a/lisp/vc/vc-hooks.el
+++ b/lisp/vc/vc-hooks.el
@@ -890,6 +890,8 @@ In the latter case, VC mode is deactivated for this buffer."
     (define-key map "L" 'vc-print-root-log)
     (define-key map "I" 'vc-log-incoming)
     (define-key map "O" 'vc-log-outgoing)
+    (define-key map "ML" 'vc-log-mergebase)
+    (define-key map "MD" 'vc-diff-mergebase)
     (define-key map "m" 'vc-merge)
     (define-key map "r" 'vc-retrieve-tag)
     (define-key map "s" 'vc-create-tag)
diff --git a/lisp/vc/vc.el b/lisp/vc/vc.el
index 0a638ec7d7f..e6f30c9f804 100644
--- a/lisp/vc/vc.el
+++ b/lisp/vc/vc.el
@@ -429,6 +429,10 @@
 ;; - region-history-mode ()
 ;;
 ;;   Major mode to use for the output of `region-history'.
+;;
+;; - mergebase (rev1 &optional rev2)
+;;
+;;   Return the common ancestor between REV1 and REV2 revisions.
 
 ;; TAG SYSTEM
 ;;
@@ -1854,6 +1858,33 @@ saving the buffer."
     (vc-diff-internal t (vc-deduce-fileset t) nil nil
                       (called-interactively-p 'interactive))))
 
+;;;###autoload
+(defun vc-diff-mergebase (_files rev1 rev2)
+  "Report diffs between the merge base of REV1 and REV2 revisions.
+The merge base is a common ancestor between REV1 and REV2 revisions."
+  (interactive
+   (vc-diff-build-argument-list-internal
+    (or (ignore-errors (vc-deduce-fileset t))
+        (let ((backend (or (vc-deduce-backend) (vc-responsible-backend default-directory))))
+          (list backend (list (vc-call-backend backend 'root default-directory)))))))
+  (when (and (not rev1) rev2)
+    (error "Not a valid revision range"))
+  (let ((backend (vc-deduce-backend))
+        (default-directory default-directory)
+        rootdir)
+    (if backend
+        (setq rootdir (vc-call-backend backend 'root default-directory))
+      (setq rootdir (read-directory-name "Directory for VC root-diff: "))
+      (setq backend (vc-responsible-backend rootdir))
+      (if backend
+          (setq default-directory rootdir)
+        (error "Directory is not version controlled")))
+    (let ((default-directory rootdir)
+          (rev1 (vc-call-backend backend 'mergebase rev1 rev2)))
+      (vc-diff-internal
+       t (list backend (list rootdir)) rev1 rev2
+       (called-interactively-p 'interactive)))))
+
 (declare-function ediff-load-version-control "ediff" (&optional silent))
 (declare-function ediff-vc-internal "ediff-vers"
                   (rev1 rev2 &optional startup-hooks))
@@ -2485,6 +2516,28 @@ When called interactively with a prefix argument, prompt for REMOTE-LOCATION."
                                    "*vc-outgoing*" 'log-outgoing)))
 
 ;;;###autoload
+(defun vc-log-mergebase (_files rev1 rev2)
+  "Show a log of changes between the merge base of REV1 and REV2 revisions.
+The merge base is a common ancestor between REV1 and REV2 revisions."
+  (interactive
+   (vc-diff-build-argument-list-internal
+    (or (ignore-errors (vc-deduce-fileset t))
+        (let ((backend (or (vc-deduce-backend) (vc-responsible-backend default-directory))))
+          (list backend (list (vc-call-backend backend 'root default-directory)))))))
+  (let ((backend (vc-deduce-backend))
+        (default-directory default-directory)
+        rootdir)
+    (if backend
+        (setq rootdir (vc-call-backend backend 'root default-directory))
+      (setq rootdir (read-directory-name "Directory for VC root-log: "))
+      (setq backend (vc-responsible-backend rootdir))
+      (unless backend
+        (error "Directory is not version controlled")))
+    (setq default-directory rootdir)
+    (setq rev1 (vc-call-backend backend 'mergebase rev1 rev2))
+    (vc-print-log-internal backend (list rootdir) rev1 t (or rev2 ""))))
+
+;;;###autoload
 (defun vc-region-history (from to)
   "Show the history of the region between FROM and TO.