object-name: don't try to abbreviate to lengths greater than hexsz

object-name: don't try to abbreviate to lengths greater than hexsz

When given a length that equals the current hash algorithm's hex size,
then `repo_find_unique_abbrev_r()` exits early without trying to find an
abbreviation. This is only sensible because there is nothing to
abbreviate in the first place, so searching through objects to find a
unique prefix would be a waste of compute.

What we don't handle though is the case where the user passes a length
greater than the hash length. This is fine in practice as we still
compute the correct result. But at the very least, this is a waste of
resources as we try to abbreviate a value that cannot be abbreviated,
which causes us to hit the object database.

Start to explicitly handle values larger than hexsz to avoid this
performance penalty, which leads to a measureable speedup. The following
benchmark has been executed in linux.git:

  Benchmark 1: git -c core.abbrev=9000 log --abbrev-commit (revision = HEAD~)
    Time (mean ± σ):     12.812 s ±  0.040 s    [User: 12.225 s, System: 0.554 s]
    Range (min … max):   12.723 s … 12.857 s    10 runs

  Benchmark 2: git -c core.abbrev=9000 log --abbrev-commit (revision = HEAD)
    Time (mean ± σ):     11.095 s ±  0.029 s    [User: 10.546 s, System: 0.521 s]
    Range (min … max):   11.037 s … 11.122 s    10 runs

  Summary
    git -c core.abbrev=9000 log --abbrev-commit HEAD (revision = HEAD) ran
      1.15 ± 0.00 times faster than git -c core.abbrev=9000 log --abbrev-commit HEAD (revision = HEAD~)

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>