[MERGE] [0.90] Restore support for Merge directive 2 / 0.19

[John Arbash Meinel]

John Arbash Meinel has voted tweak.
Status is now: Conditionally approved
Comment:
The spacing here is a little off
+    def register(self, directive, format_string=None):
+        if format_string is None:
+            format_string  = directive._format_string
+        registry.Registry.register(self, format_string, directive)

1 space, not 2.

Otherwise it seems 0.90 worthy. (Since we were using 0.19 for a while, 
so there are a few bzr.dev people who would still be using it.)


For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46BCCC8A.6080008%40utoronto.ca%3E