Fix use of mutex in threaded grep
Fix use of mutex in threaded grep

The program can decide at runtime not to use threading even if the support
is compiled in.  In such a case, mutexes are not necessary and left
uninitialized.  But the code incorrectly tried to take and release the
read_sha1_mutex unconditionally.

Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>
Acked-by: Fredrik Kuivinen <d4b884dd544d7f7cd1aa04df35b4ba0349e4e79b@gmail.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git