filemerge: in ':prompt', use ':fail' tool rather than returning directly

The ':fail' tool now knows to write out the changed side for change/delete
conflicts.

This has no impact right now but will make things better when we move
change/delete conflicts in here.

mercurial/filemerge.py

@@ -260,7 +260,7 b' def _iprompt(repo, mynode, orig, fcd, fc'
             return _ilocal(repo, mynode, orig, fcd, fco, fca, toolconf)
     except error.ResponseExpected:
         ui.write("\n")
-        return 1, False
+        return _ifail(repo, mynode, orig, fcd, fco, fca, toolconf)
 
 @internaltool('local', nomerge)
 def _ilocal(repo, mynode, orig, fcd, fco, fca, toolconf):