refs: reuse iterators when determining refname availability

When verifying whether refnames are available we have to verify whether
any reference exists that is nested under the current reference. E.g.
given a reference "refs/heads/foo", we must make sure that there is no
other reference "refs/heads/foo/*".

This check is performed using a ref iterator with the prefix set to the
nested reference namespace. Until now it used to not be possible to
reseek iterators, so we always had to reallocate the iterator for every
single reference we're about to check. This keeps us from reusing state
that the iterator may have and that may make it work more efficiently.

Refactor the logic to reseek iterators. This leads to a sizeable speedup
with the "reftable" backend:

Benchmark 1: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)
Time (mean ± σ): 39.8 ms ± 0.9 ms [User: 29.7 ms, System: 9.8 ms]
Range (min … max): 38.4 ms … 42.0 ms 62 runs

Benchmark 2: update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD)
Time (mean ± σ): 31.9 ms ± 1.1 ms [User: 27.0 ms, System: 4.5 ms]
Range (min … max): 29.8 ms … 34.3 ms 74 runs

Summary
update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD) ran
1.25 ± 0.05 times faster than update-ref: create many refs (refformat = reftable, preexisting = 100000, new = 10000, revision = HEAD~)

The "files" backend doesn't really show a huge impact:

Benchmark 1: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~)
Time (mean ± σ): 392.3 ms ± 7.1 ms [User: 59.7 ms, System: 328.8 ms]
Range (min … max): 384.6 ms … 404.5 ms 10 runs

Benchmark 2: update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD)
Time (mean ± σ): 387.7 ms ± 7.4 ms [User: 54.6 ms, System: 329.6 ms]
Range (min … max): 377.0 ms … 397.7 ms 10 runs

Summary
update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD) ran
1.01 ± 0.03 times faster than update-ref: create many refs (refformat = files, preexisting = 100000, new = 10000, revision = HEAD~)

This is mostly because it is way slower to begin with because it has to
create a separate file for each new reference, so the milliseconds we
shave off by reseeking the iterator doesn't really translate into a
significant relative improvement.

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