upload-pack.c: avoid enumerating hidden refs where possible

upload-pack.c: avoid enumerating hidden refs where possible

In a similar fashion as a previous commit, teach `upload-pack` to avoid
enumerating hidden references where possible.

Note, however, that there are certain cases where cannot avoid
enumerating even hidden references, in particular when either of:

  - `uploadpack.allowTipSHA1InWant`, or
  - `uploadpack.allowReachableSHA1InWant`

are set, corresponding to `ALLOW_TIP_SHA1` and `ALLOW_REACHABLE_SHA1`,
respectively.

When either of these bits are set, upload-pack's `is_our_ref()` function
needs to consider the `HIDDEN_REF` bit of the referent's object flags.
So we must visit all references, including the hidden ones, in order to
mark their referents with the `HIDDEN_REF` bit.

When neither `ALLOW_TIP_SHA1` nor `ALLOW_REACHABLE_SHA1` are set, the
`is_our_ref()` function considers only the `OUR_REF` bit, and not the
`HIDDEN_REF` one. `OUR_REF` is applied via `mark_our_ref()`, and only
to objects at the tips of non-hidden references, so we do not need to
visit hidden references in this case.

When neither of those bits are set, `upload-pack` can potentially avoid
enumerating a large number of references. In the same example as a
previous commit (linux.git with one hidden reference per commit,
"refs/pull/N"):

    $ printf 0000 >in
    $ hyperfine --warmup=1 \
      'git -c transfer.hideRefs=refs/pull upload-pack . <in' \
      'git.compile -c transfer.hideRefs=refs/pull -c uploadpack.allowTipSHA1InWant upload-pack . <in' \
      'git.compile -c transfer.hideRefs=refs/pull upload-pack . <in'
    Benchmark 1: git -c transfer.hideRefs=refs/pull upload-pack . <in
      Time (mean ± σ):     406.9 ms ±   1.1 ms    [User: 357.3 ms, System: 49.5 ms]
      Range (min … max):   405.7 ms … 409.2 ms    10 runs

    Benchmark 2: git.compile -c transfer.hideRefs=refs/pull -c uploadpack.allowTipSHA1InWant upload-pack . <in
      Time (mean ± σ):     406.5 ms ±   1.3 ms    [User: 356.5 ms, System: 49.9 ms]
      Range (min … max):   404.6 ms … 408.8 ms    10 runs

    Benchmark 3: git.compile -c transfer.hideRefs=refs/pull upload-pack . <in
      Time (mean ± σ):       4.7 ms ±   0.2 ms    [User: 0.7 ms, System: 3.9 ms]
      Range (min … max):     4.3 ms …   6.1 ms    472 runs

    Summary
      'git.compile -c transfer.hideRefs=refs/pull upload-pack . <in' ran
       86.62 ± 4.33 times faster than 'git.compile -c transfer.hideRefs=refs/pull -c uploadpack.allowTipSHA1InWant upload-pack . <in'
       86.70 ± 4.33 times faster than 'git -c transfer.hideRefs=refs/pull upload-pack . <in'

As above, we must visit every reference when
uploadPack.allowTipSHA1InWant is set. But when it is unset, we can visit
far fewer references.

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