After updating a set of remove refs during a 'git fetch', we walk the
commits in the new ref value and not in the old ref value to discover
if the update was a forced update. This results in two things happening
during the command:

 1. The line including the ref update has an additional "(forced-update)"
    marker at the end.

 2. The ref log for that remote branch includes a bit saying that update
    is a forced update.

For many situations, this forced-update message happens infrequently, or
is a small bit of information among many ref updates. Many users ignore
these messages, but the calculation required here slows down their fetches
significantly. Keep in mind that they do not have the opportunity to
calculate a commit-graph file containing the newly-fetched commits, so
these comparisons can be very slow.

Add a '--[no-]show-forced-updates' option that allows a user to skip this
calculation. The only permanent result is dropping the forced-update bit
in the reflog.

Include a new fetch.showForcedUpdates config setting that allows this
behavior without including the argument in every command. The config
setting is overridden by the command-line arguments.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>