For your static HTML-CSS mockups I can't see any reasons to be in the same repo, as ASP-code
- they are sources of, not independent parallel work
- Branching branch entire repo, you'll just got additional clones of single entity "current design"
LIVE and DEV versions of code fit into branching workflow a lot better and 2 independent repos is, in common, right direction - for linking changesets to Mockup-repo from ASP-repo hash of Mockup-changeset with changed design can be used in commit message of ASP-changeset