[merge][#159589] Re: LockContention has unhelpful text

[Andrew Bennetts]

Martin Pool wrote:
> This makes LockContention include the lock file name in more cases, so
> that you get some clue as to what's happening.  I reproduced this
> earlier while trying to branch from a branch while pushing it.  (I
> think that in turn is another instance of the shortcomings of
> write-locking the dirstate to update the cache while doing a readonly
> operation, but giving a better message here will make it more
> obvious.)
> 
> This includes a bonus fix for the repr of SmartClientStreamMedium; see
> the comment.

bb:tweak

It'd be good to have a test_errors test for the formatting of
LockContention.  Otherwise this looks good (and I really want that repr fix
to land!).

-Andrew.