properly detect local branches with slash (like release/19.12)

Summary:
should fix multiple builds from git with a branch with a slash into its
name by avoiding calling git checkout --track multiple times

Test Plan:
fix local build of elisa with release/19.12

see binary-factory failures

Reviewers: Windows, Craft, vonreth

Reviewed By: Windows, Craft, vonreth

Differential Revision: https://phabricator.kde.org/D25894