builtin/show-ref.c: avoid over-iterating with --heads, --tags

When `show-ref` is combined with the `--heads` or `--tags` options, it
can avoid iterating parts of a repository's references that it doesn't
care about.

But it doesn't take advantage of this potential optimization. When this
command was introduced back in 358ddb62cf (Add "git show-ref" builtin
command, 2006-09-15), `for_each_ref_in()` did exist. But since most
repositories don't have many (any?) references that aren't branches or
tags already, this makes little difference in practice.

Though for repositories with a large imbalance of branches and tags (or,
more likely in the case of server operators, many hidden references),
this can make quite a difference. Take, for example, a repository with
500,000 "hidden" references (all of the form "refs/__hidden__/N"), and
a single branch:

    git commit --allow-empty -m "base" &&
    seq 1 500000 | sed 's,\(.*\),create refs/__hidden__/\1 HEAD,' |
      git update-ref --stdin &&
    git pack-refs --all

Outputting the existence of that single branch currently takes on the
order of ~50ms on my machine. The vast majority of this time is wasted
iterating through references that we know we're going to discard.

Instead, teach `show-ref` that it can iterate just "refs/heads" and/or
"refs/tags" when given `--heads` and/or `--tags`, respectively. A few
small interesting things to note:

  - When given either option, we can avoid the general-purpose
    for_each_ref() call altogether, since we know that it won't give us
    any references that we wouldn't filter out already.

  - We can make two separate calls to `for_each_fullref_in()` (and
    avoid, say, the more specialized `for_each_fullref_in_prefixes()`,
    since we know that the set of references enumerated by each is
    disjoint, so we'll never see the same reference appear in both
    calls.

  - We have to use the "fullref" variant (instead of just
    `for_each_branch_ref()` and `for_each_tag_ref()`), since we expect
    fully-qualified reference names to appear in `show-ref`'s output.

When either of `heads_only` or `tags_only` is set, we can eliminate the
strcmp() calls in `builtin/show-ref.c::show_ref()` altogether, since we
know that `show_ref()` will never see a non-branch or tag reference.

Unfortunately, we can't use `for_each_fullref_in_prefixes()` to enhance
`show-ref`'s pattern matching, since `show-ref` patterns match on the
_suffix_ (e.g., the pattern "foo" shows "refs/heads/foo",
"refs/tags/foo", and etc, not "foo/*").

Nonetheless, in our synthetic example above, this provides a significant
speed-up ("git" is roughly v2.36, "git.compile" is this patch):

    $ hyperfine -N 'git show-ref --heads' 'git.compile show-ref --heads'
    Benchmark 1: git show-ref --heads
      Time (mean ± σ):      49.9 ms ±   6.2 ms    [User: 45.6 ms, System: 4.1 ms]
      Range (min … max):    46.1 ms …  73.6 ms    43 runs

    Benchmark 2: git.compile show-ref --heads
      Time (mean ± σ):       2.8 ms ±   0.4 ms    [User: 1.4 ms, System: 1.2 ms]
      Range (min … max):     1.3 ms …   5.6 ms    957 runs

    Summary
      'git.compile show-ref --heads' ran
       18.03 ± 3.38 times faster than 'git show-ref --heads'

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

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