Added option to keep empty branches in rebase-update.
Added option to keep empty branches in rebase-update.

Change-Id: I87b1edf242aa5c8483d8d84d223fc9187a194708
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2292962
Reviewed-by: Dirk Pranke <6693ee08b084bb2ffa767f664c99b689395e0e4b@google.com>
Commit-Queue: Allen Bauer <5ee8de6cd2c283900e79add8c2f650eb1b4be8f7@chromium.org>

CoherentLabs/depot_tools,CoherentLabs/depot_tools