buildman: Support upstream branch name containing /

Buildman assumes that branch names do not have a slash in them, since
slash is used to delimit remotes, etc. This means that a branch called
'WIP/tryme' in remote dm ends up being 'tryme'.

Adjust the logic a little, to try to accommodate this.

For now, no tests are added for this behaviour.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>