Make git map-branches faster

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>

git_common.py

@@ -1045,6 +1045,7 @@ def get_branches_info(include_tracking_status):
     (branch, branch_hash, upstream_branch, tracking_status) = line.split(':')
 
     commits = None
+    if include_tracking_status:
       base = get_or_create_merge_base(branch)
       if base:
         commits_list = run('rev-list', '--count', branch, '^%s' % base, '--')