SCM Git: add better support for git, and scm_subdir

Check for the git tag "r" number.

Also add support for new INI param: scm_subdir.  Since we can't check out
a subset of a repo in git, allow specifying a scm_subdir to tell MTT to
clone the whole repo and then cd into <scm_subdir> (thereby effectively
emulating checking out a subset of the repo).