tmt should log and expose git refs it clones

[happz]

In its simplest workflow, tmt is executed in a directory with tests, plans, and stories. That's fine. Then the test becomes a git repository, with a history of changes, and then tests become more complicated and start using beaker libraries, and then plans start referencing remote plans from other git repositories, and so on. Users interested in reproducible testing may (and do) ask a question of what plans and tests - exactly, please - were used in a given test run, and tmt should be more helpful by logging and exposing refs of various repositories it encounters.

The implementation may as simple as extending tmt.utils.git_clone() by adding git rev-parse HEAD & logging the output. It might be good enough to start with, but eventually, tmt could provide this info in a more structured way, e.g. via a dedicated YAML file.

Related to https://issues.redhat.com/browse/TFT-3050