Fix interaction between vc-hg find-file-hook and vc state caching

Bad assumptions in the `vc-hg-find-file-hook' prevented it from
working. This correctly them.  (Bug#33129).

2018-10-23  Daniel Pittman  <slippycheeze@google.com>

	* lisp/vc/vc-hg.el (vc-hg-find-file-hook): This function made two
	assumptions about conflicted files that were not accurate,
	preventing conflicts in files ever being detected.

	The first was that the `vc-state' was cache by the time this was
	invoked, which it is not - at least when visiting the file, or
	using `vc-refresh-state'.

	The second was that a file with the ".orig" extension would be
	present, next to the file being visited.  This is the default
	behavior of Mercurial, but can be overridden by the user.

	Since the VC mode-line code will shortly calculate the state for
	display, the optimization of testing for the ".orig" file only
	delayed this work by a few moments.

2 files changed, 7 insertions, 3 deletions

diff --git a/etc/NEWS b/etc/NEWS
index ac23b3b1814..b71ac804e39 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -339,6 +339,12 @@ When no files are marked, all modified files are stashed, as before.
 
 *** The new hook 'vc-retrieve-tag-hook' runs after retrieving a tag.
 
+--- `vc-hg' now invokes `smerge-mode' correctly when visiting files.
+Code that attempted to invoke `smerge-mode' when visiting an Hg file
+with conflicts existed in earlier versions of Emacs, but incorrectly
+never detected a conflict due to invalid assumptions about cached
+values.
+
 ** diff-mode
 *** Hunks are now automatically refined by default.
 To disable it, set the new defcustom 'diff-font-lock-refine' to nil.
diff --git a/lisp/vc/vc-hg.el b/lisp/vc/vc-hg.el
index 36965735959..d528813bc08 100644
--- a/lisp/vc/vc-hg.el
+++ b/lisp/vc/vc-hg.el
@@ -1142,11 +1142,9 @@ REV is the revision to check out into WORKFILE."
 
 (defun vc-hg-find-file-hook ()
   (when (and buffer-file-name
-             (file-exists-p (concat buffer-file-name ".orig"))
              ;; Hg does not seem to have a "conflict" status, eg
              ;; hg http://bz.selenic.com/show_bug.cgi?id=2724
-             (memq (vc-file-getprop buffer-file-name 'vc-state)
-                   '(edited conflict))
+             (memq (vc-state buffer-file-name) '(edited conflict))
              ;; Maybe go on to check that "hg resolve -l" says "U"?
              ;; If "hg resolve -l" says there's a conflict but there are no
              ;; conflict markers, it's not clear what we should do.