[MERGE][136890] ``merge --uncommitted`` with a path to a specific file should merge only that file

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
Please put the fix into Merger.from_uncommitted.  It's cleaner that way.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1189162119.8945.3.camel%40nemo%3E