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 <gitster@pobox.com> Acked-by: Fredrik Kuivinen <frekui@gmail.com>