git-codereview: fix active rebase detection
git-codereview: fix active rebase detection

A sync + merge conflict + git rebase --continue can leave behind
a REBASE_HEAD even though it really should not. So don't use that
as the signal that there's an active rebase.

The rebase-merge directory is a better signal and seems to be
cleaned up more diligently than REBASE_HEAD.

Change-Id: Ie5bc20156e19fc7ce3e32333babea784242dc770
Reviewed-on: https://go-review.googlesource.com/c/review/+/293838
Trust: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Run-TryBot: Russ Cox <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
Reviewed-by: Katie Hockman <e321a8fd89c4908465ddaf8d5d0d62480efacb63@golang.org>

golang/review