Pass filename to $HGMERGE

This lets the merge program be slightly more user-friendly

mercurial/hg.py

@@ -776,7 +776,7 b' class localrepository:'
 
             cmd = os.environ["HGMERGE"]
             self.ui.debug("invoking merge with %s\n" % cmd)
-            r = os.system("%s %s %s %s" % (cmd, a, b, c))
+            r = os.system("%s %s %s %s %s" % (cmd, a, b, c, fn))
             if r:
                 raise "Merge failed!"