bundle: fix non-linear performance scaling with refs

The 'git bundle create' command has non-linear performance with the
number of refs in the repository. Benchmarking the command shows that
a large portion of the time (~75%) is spent in the
`object_array_remove_duplicates()` function.

The `object_array_remove_duplicates()` function was added in
b2a6d1c686 (bundle: allow the same ref to be given more than once,
2009-01-17) to skip duplicate refs provided by the user from being
written to the bundle. Since this is an O(N^2) algorithm, in repos with
large number of references, this can take up a large amount of time.

Let's instead use a 'strset' to skip duplicates inside
`write_bundle_refs()`. This improves the performance by around 6 times
when tested against in repository with 100000 refs:

Benchmark 1: bundle (refcount = 100000, revision = master)
Time (mean ± σ): 14.653 s ± 0.203 s [User: 13.940 s, System: 0.762 s]
Range (min … max): 14.237 s … 14.920 s 10 runs

Benchmark 2: bundle (refcount = 100000, revision = HEAD)
Time (mean ± σ): 2.394 s ± 0.023 s [User: 1.684 s, System: 0.798 s]
Range (min … max): 2.364 s … 2.425 s 10 runs

Summary
bundle (refcount = 100000, revision = HEAD) ran
6.12 ± 0.10 times faster than bundle (refcount = 100000, revision = master)

Previously, `object_array_remove_duplicates()` ensured that both the
refname and the object it pointed to were checked for duplicates. The
new approach, implemented within `write_bundle_refs()`, eliminates
duplicate refnames without comparing the objects they reference. This
works because, for bundle creation, we only need to prevent duplicate
refs from being written to the bundle header. The `revs->pending` array
can contain duplicates of multiple types.

First, references which resolve to the same refname. For e.g. "git
bundle create out.bdl master master" or "git bundle create out.bdl
refs/heads/master refs/heads/master" or "git bundle create out.bdl
master refs/heads/master". In these scenarios we want to prevent writing
"refs/heads/master" twice to the bundle header. Since both the refnames
here would point to the same object (unless there is a race), we do not
need to check equality of the object.

Second, refnames which are duplicates but do not point to the same
object. This can happen when we use an exclusion criteria. For e.g. "git
bundle create out.bdl master master^!", Here `revs->pending` would
contain two elements, both with refname set to "master". However, each
of them would be pointing to an INTERESTING and UNINTERESTING object
respectively. Since we only write refnames with INTERESTING objects to
the bundle header, we perform our duplicate checks only on such objects.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>