If a local branch tracks removed remote tracking branch (e.g. foo) and
remote has a branch that starts with such name (e.g. foo/bar), git fetch
will fail. --prune flag removes any remote-tracking branches that no
longer exist.

R=apolito@google.com, ehmaldonado@chromium.org

Bug: 1079483
Change-Id: I9bc31bf961d52a86b6fa2342249971b99a003666
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2190341
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>