Running git rebase --continue may launch a text editor for editing commit messages. This causes testRebase to hang or to fail completely when running locally.
Setting GIT_EDITOR to ':' suppresses this.

Bug:1149625
Change-Id: Ia4a0b6564b198be945866cb890607ac8966c468f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2551375
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>