Merges the given rev using the given message. Like test_commit,
creates a tag and calls test_tick before committing.
+ - test_must_fail <git-command>
+ Run a git command and ensure it fails in a controlled way. Use
+ this instead of "! <git-command>". When git-command dies due to a
+ segfault, test_must_fail diagnoses it as an error; "! <git-command>"
+ treats it as just another expected failure, which would let such a