mergestate: determine if active without looking for state files on disk

I couldn't think of a reason that we need to check state files on disk
to determine if a merge is active. I could imagine them being for
there for detecting broken state files that would then be cleaned up
by some later command, but we always delete the entire `.hg/merge/`
tree, so that doesn't seem to be it.

The checks were added in 4e932dc5c113 (resolve: abort when not
applicable (BC), 2014-04-18). Perhaps there were needed for that and
then made obsolete by 6062593d8b06 (resolve: don't abort resolve -l
even when no merge is in progress, 2014-05-23).

The reason I want to delete the checks is that I think `ms =
mergestate.read(repo); ms.active() and ms.local` should be a valid
pattern, but it crashes when the merge state file is an empty file if
we consider mere presence of the file as "active".

Differential Revision: https://phab.mercurial-scm.org/D8118

mercurial/merge.py

@@ -413,14 +413,7 b' class mergestate(object):'
         Returns True if there appears to be mergestate. This is a rough proxy
         for "is a merge in progress."
         """
-        # Check local variables before looking at filesystem for performance
-        # reasons.
-        return (
-            bool(self._local)
-            or bool(self._state)
-            or self._repo.vfs.exists(self.statepathv1)
-            or self._repo.vfs.exists(self.statepathv2)
-        )
+        return bool(self._local) or bool(self._state)
 
     def commit(self):
         """Write current state on disk (if necessary)"""