[MERGE] Small cmd_diff fixes

[Alexander Belchenko]

Alexander Belchenko has voted +1 (conditional).
Status is now: Semi-approved
Comment:
I'd like to see blackbox test for incorrect --prefix value, to test rhis 
code path:

- raise BzrCommandError(
+ raise errors.BzrCommandError(
"--prefix expects two values separated by a colon")

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45EFB5D0.40705%40hlabs.spb.ru%3E