[MERGE] Single lock for 'bzr diff'

[John Arbash Meinel]

John Arbash Meinel has voted resubmit.
Status is now: Resubmit
Comment:
It seems that doing this exposes diff to the 'bzr diff -r branch:...' 
bug that 'bzr status' has.
Specifically, we are grabbing a read lock early, and then branch: tries 
to do a fetch and fails with a big traceback.

And we *do* have a test that 'bzr diff -r branch:...' works, so a test 
actually fails.

So while I think branch: is what is broken, it means this cannot be 
merged as is.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46FBE34A.1060905%40arbash-meinel.com%3E