builtin/repack.c: avoid using `hash_to_hex()` in pack geometry

builtin/repack.c: avoid using `hash_to_hex()` in pack geometry

In previous commits, we started passing either repository or
git_hash_algo pointers around to various spots within builtin/repack.c
to reduce our dependency on the_repository in the hope of undef'ing
USE_THE_REPOSITORY_VARIABLE.

This commit takes us as far as we can (easily) go in that direction by
removing the only use of a convenience function that only exists when
USE_THE_REPOSITORY_VARIABLE is defined.

Unfortunately, the only other such function is "is_bare_repository()",
which is less than straightforward to convert into, say,
"repo_is_bare()", the latter of the two accepting a repository pointer.

Punt on that for now, and declare this commit as the stopping point for
our efforts in the direction of undef'ing USE_THE_REPOSITORY_VARIABLE.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>