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 <wmpalmer@gmail.com>
Signed-off-by: Stephen P. Smith <ischis2@cox.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t1511-rev-parse-caret.sh

@@ -18,7 +18,15 @@ test_expect_success 'setup' '
 	git checkout master &&
 	echo modified >>a-blob &&
 	git add -u &&
-	git commit -m Modified
+	git commit -m Modified &&
+	git branch modref &&
+	echo changed! >>a-blob &&
+	git add -u &&
+	git commit -m !Exp &&
+	git branch expref &&
+	echo changed >>a-blob &&
+	git add -u &&
+	git commit -m Changed
 '
 
 test_expect_success 'ref^{non-existent}' '
@@ -77,4 +85,18 @@ test_expect_success 'ref^{/Initial}' '
 	test_cmp expected actual
 '
 
+test_expect_success 'ref^{/!Exp}' '
+	test_must_fail git rev-parse master^{/!Exp}
+'
+
+test_expect_success 'ref^{/!}' '
+	test_must_fail git rev-parse master^{/!}
+'
+
+test_expect_success 'ref^{/!!Exp}' '
+	git rev-parse expref >expected &&
+	git rev-parse master^{/!!Exp} >actual &&
+	test_cmp expected actual
+'
+
 test_done