vc-hg-working-revision: Return changeset hashes

* lisp/vc/vc-hg.el (vc-hg-working-revision): Return changeset
hashes instead of revision numbers.  Respect
vc-use-short-revision.
* etc/NEWS: Document the change.

etc/NEWS

@@ -3088,6 +3088,11 @@ You can use 'C-u C-x v +' to preview or change the arguments.
 *** 'C-u C-x v +' and 'C-u C-x v P' for Git have an input history.
 This was already in place for Mercurial.
 
+---
+*** 'vc-hg-working-revision' now returns changeset hashes.
+Previously it returned local revision numbers, but hashes are more
+robust for how this function is typically used.
+
 ** Package
 
 +++

lisp/vc/vc-hg.el

@@ -282,7 +282,8 @@ A value of `default' means to use the value of `vc-resolve-conflicts'."
   (ignore-errors
     (with-output-to-string
       (vc-hg-command standard-output 0 nil
-                     "log" "-r" "." "--template" "{rev}"))))
+                     "log" "-r" "." "--template"
+                     (if vc-use-short-revision "{node|short}" "{node}")))))
 
 (defcustom vc-hg-symbolic-revision-styles
   '(builtin-active-bookmark