localrepository.status: only acquire wlock if actually needed.

This speeds up the common case of not needing to update the dirstate,
and avoids the need to reload and parse the dirstate "just in case".

mercurial/localrepo.py

@@ -919,13 +919,10 b' class localrepository(repo.repository):'
             # all the revisions in parent->child order.
             mf1 = mfmatches(node1)
 
+        mywlock = False
+
         # are we comparing the working directory?
         if not node2:
-            if not wlock:
-                try:
-                    wlock = self.wlock(wait=0)
-                except lock.LockException:
-                    wlock = None
             (lookup, modified, added, removed, deleted, unknown,
              ignored, clean) = self.dirstate.status(files, match,
                                                     list_ignored, list_clean)
@@ -942,7 +939,13 b' class localrepository(repo.repository):'
                             modified.append(f)
                         else:
                             clean.append(f)
-                            if wlock is not None:
+                            if not wlock and not mywlock:
+                                mywlock = True
+                                try:
+                                    wlock = self.wlock(wait=0)
+                                except lock.LockException:
+                                    pass
+                            if wlock:
                                 self.dirstate.update([f], "n")
             else:
                 # we are comparing working dir against non-parent
@@ -957,6 +960,9 b' class localrepository(repo.repository):'
                 for f in removed:
                     if f in mf2:
                         del mf2[f]
+
+            if mywlock and wlock:
+                wlock.release()
         else:
             # we are comparing two revisions
             mf2 = mfmatches(node2)