Tagging during a rebase doesn't make sense, but it at least should
not choke with:
$ ./build-aux/do-release-commit-and-tag 1.4.18 stable
./build-aux/do-release-commit-and-tag: line 146: test: too many arguments
do-release-commit-and-tag: not on branch (no branch, rebasing branch-1.4)
* build-aux/do-release-commit-and-tag (curr_br): $branch can
contain spaces when rebasing.