bzr shelve/unshelve should test the availability of diffstat before running it

[Matthieu Moy]

On a system without diffstat installed, I get

$ bzr unshelve
Reapplying shelved patches 
Diff status is now:
sh: line 1: diffstat: command not found
broken pipe
$ 

I'd expect for example

$ bzr unshelve
Reapplying shelved patches 
Warning: diffstat not available. Can not compute diff status.
$ 

(This "broken pipe" message may mean bzr died unexpectedly while it
didn't finish the job)

-- 
Matthieu