If user downloaded depot_tools.zip before 2021-03-11, it contained only
fetch for refs/heads/master. As main branch is a new default branch,
users need to update fetch ref in order to fetch chanes from main
branch.

R=ehmaldonado@google.com

Bug: 1187238
Change-Id: I78041fc74f1bb505db375b9c1dced38cdfc9b144
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2753834Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>