localrepo: use commonancestorsheads for checking linear heritage in file commit

If two revisions are linearly related, there will only be one ancestor, and
commonancestors and commonancestorsheads would give the same result.
commonancestorsheads is however slightly simpler, faster and more correct.

mercurial/localrepo.py

@@ -1168,7 +1168,7 b' class localrepository(object):'
             fparent1, fparent2 = fparent2, nullid
         elif fparent2 != nullid:
             # is one parent an ancestor of the other?
-            fparentancestors = flog.commonancestors(fparent1, fparent2)
+            fparentancestors = flog.commonancestorsheads(fparent1, fparent2)
             if fparent1 in fparentancestors:
                 fparent1, fparent2 = fparent2, nullid
             elif fparent2 in fparentancestors: