Since commit 8eca0b47, it is possible
for read_sha1_file() to return NULL even with existing objects when they
are corrupted.  Previously a corrupted object would have terminated the
program immediately, effectively making read_sha1_file() return NULL
only when specified object is not found.

Let's restore this behavior for all users of read_sha1_file() and
provide a separate function with the ability to not terminate when
bad objects are encountered.
Signed-off-by: NNicolas Pitre <nico@cam.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>