mergestate: make in-memory mergestate not clear on-disk mergestate on reset()

Oops, I thought I had remove the `rmtree()` call earlier. Maybe I
accidentally got it back in a histedit or something.

Differential Revision: https://phab.mercurial-scm.org/D9042

mercurial/mergestate.py

@@ -193,7 +193,7 b' class _mergestate_base(object):'
         self._dirty = False
 
     def reset(self):
-        shutil.rmtree(self._repo.vfs.join(b'merge'), True)
+        pass
 
     def start(self, node, other, labels=None):
         self._local = node