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
b2a6d1c6 (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 a 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)

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>