fetch: warn about forced updates in branch listing

fetch: warn about forced updates in branch listing

The --[no-]show-forced-updates option in 'git fetch' can be confusing
for some users, especially if it is enabled via config setting and not
by argument. Add advice to warn the user that the (forced update)
messages were not listed.

Additionally, warn users when the forced update check takes longer
than ten seconds, and recommend that they disable the check. These
messages can be disabled by the advice.fetchShowForcedUpdates config
setting.

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