midx.c: consider annotated tags during bitmap selection

midx.c: consider annotated tags during bitmap selection

When generating a multi-pack bitmap without a `--refs-snapshot` (e.g.,
by running `git multi-pack-index write --bitmap` directly), we determine
the set of bitmap-able commits by enumerating each reference, and adding
the referrent as the tip of a reachability traversal when it appears
somewhere in the MIDX. (Any commit we encounter during the reachability
traversal then becomes a candidate for bitmap selection).

But we incorrectly avoid peeling the object at the tip of each
reference. So if we see some reference that points at an annotated tag
(which in turn points through zero or more additional annotated tags at
a commit), that we will not add it as a tip for the reachability
traversal. This means that if some commit C is only referenced through
one or more annotated tag(s), then C won't become a bitmap candidate.

Correct this by peeling the reference tips as we enumerate them to
ensure that we consider commits which are the targets of annotated tags,
in addition to commits which are referenced directly.

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