test for '!' handling in rev-parse's named commits

test for '!' handling in rev-parse's named commits

In anticipation of extending this behaviour, add tests verifying the
handling of exclamation marks when looking up a commit "by name".

Specifically, as documented: '<rev>^{/!Message}' should fail, as the '!'
prefix is reserved; while '<rev>^{!!Message}' should search for a commit
whose message contains the string "!Message".

Signed-off-by: Will Palmer <redacted>
Signed-off-by: Stephen P. Smith <redacted>
Signed-off-by: Junio C Hamano <redacted>