scmutil: extract function that builds revset expr to select bookmark branch
This is needed to process "log -B" option properly. "log" options have to
be translated to a revset expression, not to an evaluated set.
Set up
$ hg init repo
$ cd repo
Try to import an empty patch
$ hg import --no-commit - <<EOF
> EOF
applying patch from stdin
abort: stdin: no diffs found
[10]
No dirstate backups are left behind
$ ls .hg/dirstate* | sort
.hg/dirstate