Publicize behavior of log-view-diff at beginning/end of active region

* lisp/vc/log-view.el (log-view-diff, log-view-diff-changeset):
* doc/emacs/maintaining.texi (VC Change Log): Document behavior of
log-view-diff at the beginning and the end of the log buffer
when the region is active.  (Bug#35624)

* lisp/vc/vc-git.el (vc-git-print-log): Insert newline at the
beginning to enable the feature of diffing with the working revision.

1 files changed, 11 insertions, 0 deletions

diff --git a/doc/emacs/maintaining.texi b/doc/emacs/maintaining.texi
index 4986c111030..0fbf91ae613 100644
--- a/doc/emacs/maintaining.texi
+++ b/doc/emacs/maintaining.texi
@@ -1033,6 +1033,7 @@ systems support modifying change comments.
 Visit the revision indicated at the current line.
 
 @item d
+@itemx =
 Display a diff between the revision at point and the next earlier
 revision, for the specific file.
 
@@ -1047,6 +1048,16 @@ L}), toggle between showing and hiding the full log entry for the
 revision at point.
 @end table
 
+To compare two arbitrary revisions, activate the region: set the
+beginning of the region to the line with the first revision and the
+end of the region to the line with the second revision to compare,
+then type @kbd{d} or @kbd{=}.  When the beginning of the region is on
+the top line that has no revision, it uses the current working revision
+as the first revision to compare.  When the end of the region is on
+the bottom non-revision line after the last revision line, then it
+uses the next earlier revision after the last displayed revision as
+the second revision to compare.
+
 @vindex vc-log-show-limit
 Because fetching many log entries can be slow, the
 @file{*vc-change-log*} buffer displays no more than 2000 revisions by