ExternalProject: Avoid unnecessary git clones (#12564)

In the case of git, only track the repository in the
repository info dependency tracking file. Not the tag.

The download step should only re-run if the repository changes.
The download step should NOT re-run if the tag changes.

The update step is an 'always' re-running step, and so should
already re-run, unless it's been eliminated by use of

  UPDATE_COMMAND ""