fetch: avoid unpacking headers in object existence check

When updating local refs after the fetch has transferred all objects, we
do an object existence test as a safety guard to avoid updating a ref to
an object which we don't have. We do so via `oid_object_info()`: if it
returns an error, then we know the object does not exist.

One side effect of `oid_object_info()` is that it parses the object's
type, and to do so it must unpack the object header. This is completely
pointless: we don't care for the type, but only want to assert that the
object exists.

Refactor the code to use `repo_has_object_file()`, which both makes the
code's intent clearer and is also faster because it does not unpack
object headers. In a real-world repo with 2.3M refs, this results in a
small speedup when doing a mirror-fetch:

    Benchmark #1: HEAD~: git-fetch
      Time (mean ± σ):     33.686 s ±  0.176 s    [User: 30.119 s, System: 5.262 s]
      Range (min … max):   33.512 s … 33.944 s    5 runs

    Benchmark #2: HEAD: git-fetch
      Time (mean ± σ):     31.247 s ±  0.195 s    [User: 28.135 s, System: 5.066 s]
      Range (min … max):   30.948 s … 31.472 s    5 runs

    Summary
      'HEAD: git-fetch' ran
        1.08 ± 0.01 times faster than 'HEAD~: git-fetch'

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

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