[MERGE] Lock working trees inside cmd_diff to avoid multiple locks and unlocks

[Aaron Bentley]

Aaron Bentley has voted resubmit.
Status is now: Resubmit
Comment:
This doesn't pass the test suite, so we can't merge it even if we wanted 
to.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C1196442506.6389.20.camel%40laptop%3E