[patch] improve 'commit DIR'

[Aaron Bentley]

Martin Pool wrote:
> This fixes an edge case of the current commit command: you cannot say
> 'bzr commit DIR' if there is a pending merge in DIR.  It's not super
> important but is useful in writing another test.

There's another edge case though:
bzr commit FILE, where only that file has changed.  We could solve both 
cases at once by seeing if the list of changed files was the same as the 
list of selected files (after adding children of directories).

Aaron