hg merge: abort if there are outstanding changes in the working directory

We currently don't support merging from the tip into the working
directory, so merge with outstanding local changes is asking for
trouble.

hg

@@ -406,6 +406,11 b' elif cmd == "debugindexdot":'
     print "}"
 
 elif cmd == "merge":
+    (c, a, d) = repo.diffdir(repo.root, repo.current)
+    if c:
+        ui.warn("aborting (outstanding changes in working directory)\n")
+        sys.exit(1)
+
     if args:
         other = hg.repository(ui, args[0])
         ui.status("requesting changegroup\n")