This change makes git map-branches a little bit faster by avoiding
fetching the revision list of each branch if git map-branches will not
show the tracking info anyway.

Change-Id: I47458871f904004f910aadd7d774bea5193c979e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2695393Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Antonio Sartori <antoniosartori@chromium.org>