x/review/git-codereview: use rebase branch during rebase

[aclements]

Currently git-codereview doesn't add the appropriate "[branch]" prefix to commit messages created or amended during a rebase because all it sees is the detached HEAD state. As a fall back, it could look for .git/rebase-merge/head-name, which contains the full ref name of the branch being rebased and find that branch's upstream.

For example, this happened in https://golang.org/cl/50875.

/cc @josharian

[gopherbot]

Change https://golang.org/cl/375174 mentions this issue: git-codereview: hook: fixCommitMessage: improve upstream branch detection