tests: Fix git help spewage when running from releases
tests: Fix git help spewage when running from releases

If there is no .git directory, i.e. when running from a release tarball,
then run-tests spits out an unsightly two-page long git error and help.
Check if the project is a git checkout before trying to determine
changed tests.

garrett/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,mvollmer/cockpit,cockpit-project/cockpit,garrett/cockpit,martinpitt/cockpit,martinpitt/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,mvollmer/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit