Fix testRebase

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>

tests/git_common_test.py

@@ -36,6 +36,7 @@ class GitCommonTestBase(unittest.TestCase):
     import git_common
     cls.gc = git_common
     cls.gc.TEST_MODE = True
+    os.environ["GIT_EDITOR"] = ":" # Supress git editor during rebase.
 
 
 class Support(GitCommonTestBase):