Implement tag and revision options for git modules.

This makes the 'tag' option allow for checking out a given git tag for a
module (which is enforced by using refs/tags/$foo when passing to git).

Additionally, the 'revision' option has been adopted for git modules as
well, to allow for checking out any other git "tree-ish" that git
supports (i.e. anything supported by 'git checkout $foo').

Both options should leave the source directory in a detached HEAD state,
although kdesrc-build will adapt as necessary with each source update.

The auto-stashing is still attempted for both types, and a failure to
apply an auto-stashed change will cause the module to fail to build.
Ensure you're in a clean working directory if you want to avoid issues
due to this.

The test suite continues to pass (not that it's very extensive).

BUG:308493
FIXED-IN:1.16