[MERGE] Cleanup and test Lock objects

[Martin Pool]

Martin Pool has voted +1.
Status is now: Approved
Comment:
+    _fmt = "Cannot acquire write lock on %(fname)s. File is readonly."

Adding a specific exception class sounds good.  I think you could get 
eaccess for other reasons when the file is not readonly?  Maybe it would 
be better to include the message from the IOError.

I agree with Alexander that we should perhaps have per-implementation 
tests.

Incidentally: maybe the tests should be named on the pattern of 
tests/per_*, like per_lock, per_workingtree.  Although the test 
parameterization mechanism is very cool, I think it may be slightly 
confusing that/why not all the workingtree implementation tests are in 
that directory.  (And the names are rather unwieldy.)

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C45F731B8.2000603%40arbash-meinel.com%3E