fetch: skip computing output width when not printing anything

fetch: skip computing output width when not printing anything

When updating references via git-fetch(1), then by default we report to
the user which references have been changed. This output is formatted in
a nice table such that the different columns are aligned. Because the
first column contains abbreviated object IDs we thus need to iterate
over all refs which have changed and compute the minimum length for
their respective abbreviated hashes. While this effort makes sense in
most cases, it is wasteful when the user passes the `--quiet` flag: we
don't print the summary, but still compute the length.

Skip computing the summary width when the user asked for us to be quiet.
This gives us a speedup of nearly 10% when doing a mirror-fetch in a
repository with thousands of references being updated:

    Benchmark 1: git fetch --quiet +refs/*:refs/* (HEAD~)
      Time (mean ± σ):     96.078 s ±  0.508 s    [User: 91.378 s, System: 10.870 s]
      Range (min … max):   95.449 s … 96.760 s    5 runs

    Benchmark 2: git fetch --quiet +refs/*:refs/* (HEAD)
      Time (mean ± σ):     88.214 s ±  0.192 s    [User: 83.274 s, System: 10.978 s]
      Range (min … max):   87.998 s … 88.446 s    5 runs

    Summary
      'git fetch --quiet +refs/*:refs/* (HEAD)' ran
        1.09 ± 0.01 times faster than 'git fetch --quiet +refs/*:refs/* (HEAD~)'

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