grep: Fix race condition in delta_base_cache

grep: Fix race condition in delta_base_cache

When running large git grep (ie: git grep regexp $(git rev-list --all)), glibc error sometimes occur:
*** glibc detected *** git: double free or corruption (!prev): 0x00000000010abdf0 ***

According to gdb the problem originate from release_delta_cash (sha1_file.c:1703)
		free(ent->data);

>From my analysis it seems that git grep threads do acquire lock before calling read_sha1_file but not before calling
read_object_with_reference who ends up calling read_sha1_file too.

Adding the lock around read_object_with_reference seems to fix the issue for me.
I've ran git grep about a dozen time and seen no more error while
it usually happened half the time before.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas@morey-chaisemartin.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>