Reverted commit 4511b131 had a bug where
squash_ret.success = True and empty_rebase = False. In that case, stdout
and stderr would already be string and trying to decode would fail.

Bug: 1071280
Change-Id: Iadcc526147ebb98aa7a91a7daa64ef44066e83b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2163387
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>