grep: move sha1-reading mutex into low-level code

grep: move sha1-reading mutex into low-level code

The multi-threaded git-grep code needs to serialize access
to the thread-unsafe read_sha1_file call. It does this with
a mutex that is local to builtin/grep.c.

Let's instead push this down into grep.c, where it can be
used by both builtin/grep.c and grep.c. This will let us
safely teach the low-level grep.c code tricks that involve
reading from the object db.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>