fetch: avoid lookup of commits when not appending to FETCH_HEAD

When fetching from a remote repository we will by default write what has
been fetched into the special FETCH_HEAD reference. The order in which
references are written depends on whether the reference is for merge or
not, which, despite some other conditions, is also determined based on
whether the old object ID the reference is being updated from actually
exists in the repository.

To write FETCH_HEAD we thus loop through all references thrice: once for
the references that are about to be merged, once for the references that
are not for merge, and finally for all references that are ignored. For
every iteration, we then look up the old object ID to determine whether
the referenced object exists so that we can label it as "not-for-merge"
if it doesn't exist. It goes without saying that this can be expensive
in case where we are fetching a lot of references.

While this is hard to avoid in the case where we're writing FETCH_HEAD,
users can in fact ask us to skip this work via `--no-write-fetch-head`.
In that case, we do not care for the result of those lookups at all
because we don't have to order writes to FETCH_HEAD in the first place.

Skip this busywork in case we're not writing to FETCH_HEAD. The
following benchmark performs a mirror-fetch in a repository with about
two million references via `git fetch --prune --no-write-fetch-head
+refs/*:refs/*`:

    Benchmark 1: HEAD~
      Time (mean ± σ):     75.388 s ±  1.942 s    [User: 71.103 s, System: 8.953 s]
      Range (min … max):   73.184 s … 76.845 s    3 runs

    Benchmark 2: HEAD
      Time (mean ± σ):     69.486 s ±  1.016 s    [User: 65.941 s, System: 8.806 s]
      Range (min … max):   68.864 s … 70.659 s    3 runs

    Summary
      'HEAD' ran
        1.08 ± 0.03 times faster than 'HEAD~'

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

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