pack-bitmap: factor out 'bitmap_for_commit()'

pack-bitmap: factor out 'bitmap_for_commit()'

A couple of callers within pack-bitmap.c duplicate logic to lookup a
given object id in the bitamps khash. Factor this out into a new
function, 'bitmap_for_commit()' to reduce some code duplication.

Make this new function non-static, since it will be used in later
commits from outside of pack-bitmap.c.

Signed-off-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>