[merge] add lock debugging, remove unnecessary lock peeks

[Martin Pool]

This patch adds a global -Dlock option that helps you profile locking,
to make it more obvious when locks are being taken unnecessarily or
repeatedly.  By adding it I found that we are unnecessarily reading
back the lock twice while acquiring it.  I was hoping to actually find
more - haven't yet, which is good, but maybe we will later.

I might also add similar tracing to OS locks or dirstate if this is accepted.

This also

 * renames _base_Lock to _OSLock, as a better name -- i was thinking
of adding an overall BaseLock class
 * makes the assertContainsRe failure message a bit more readable when
the string being grepped is long, which it often is

-- 
Martin
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