ref-filter: avoid parsing tagged objects in match_points_at()

ref-filter: avoid parsing tagged objects in match_points_at()

When we peel tags to check if they match a --points-at oid, we
recursively parse the tagged object to see if it is also a tag. But
since the tag itself tells us the type of the object it points to (and
even gives us the appropriate object struct via its "tagged" member), we
can use that directly.

We do still have to make sure to call parse_tag() before looking at each
tag. This is redundant for the outermost tag (since we did call
parse_object() to find its type), but that's OK; parse_tag() is smart
enough to make this a noop when the tag has already been parsed.

In my clone of linux.git, with 782 tags (and only 3 non-tags), this
yields a significant speedup (bringing us back where we were before the
commit before this one started recursively dereferencing tags):

  Benchmark 1: ./git.old for-each-ref --points-at=HEAD --format="%(refname)"
    Time (mean ± σ):      20.3 ms ±   0.5 ms    [User: 11.1 ms, System: 9.1 ms]
    Range (min … max):    19.6 ms …  21.5 ms    141 runs

  Benchmark 2: ./git.new for-each-ref --points-at=HEAD --format="%(refname)"
    Time (mean ± σ):      11.4 ms ±   0.2 ms    [User: 6.3 ms, System: 5.0 ms]
    Range (min … max):    11.0 ms …  12.2 ms    250 runs

  Summary
    './git.new for-each-ref --points-at=HEAD --format="%(refname)"' ran
      1.79 ± 0.05 times faster than './git.old for-each-ref --points-at=HEAD --format="%(refname)"'

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>