git grep: be careful to use mutexes only when they are initialized

git grep: be careful to use mutexes only when they are initialized

Rather nasty things happen when a mutex is not initialized but locked
nevertheless. Now, when we're not running in a threaded manner, the mutex
is not initialized, which is correct. But then we went and used the mutex
anyway, which -- at least on Windows -- leads to a hard crash (ordinarily
it would be called a segmentation fault, but in Windows speak it is an
access violation).

This problem was identified by our faithful tests when run in the msysGit
environment.

To avoid having to wrap the line due to the 80 column limit, we use
the name "WHEN_THREADED" instead of "IF_USE_THREADS" because it is one
character shorter. Which is all we need in this case.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>