[MERGE] don't show pending merges when given specific files

[Alexander Belchenko]

John Arbash Meinel пишет:
> This is just a simple workaround for how slow "bzr status foo" can be 
> after a merge.

Yeah, I notice this slowness many times. It bugging me every time I do merge.
Even so my trees is much smaller than really big trees. So I think it's
until we have more deep fix for this this patch is OK.
bb:approve