Don't mark files with 'm'erge unless we're actually merging branches

For 3-way merges, we were marking files as 'm', which should only be
used for branch merges. This was causing files to show up in 'hg
status' that aren't actually modified.

mercurial/hg.py

@@ -1507,7 +1507,7 b' class localrepository:'
             self.merge3(f, m, o)
             util.set_exec(self.wjoin(f), flag)
             if moddirstate:
-                self.dirstate.update([f], 'm')
+                self.dirstate.update([f], mode)
 
         remove.sort()
         for f in remove: