Add tests for wildcard "path vs ref" disambiguation

Commit 28fcc0b (pathspec: avoid the need of "--" when wildcard is used -
2015-05-02) changes how the disambiguation rules work. This patch adds
some tests to demonstrate, basically, if wildcard characters are in an
argument:

 - if the argument is valid extended sha-1 syntax, "--" must be used
 - otherwise the argument is considered a path, even without "--"

And wildcard can appear in extended sha-1 syntax, either as part of
regex in ":/<regex>" or as the literal path in ":<path>". The latter
case is less likely to happen in real world. But if you do ":/" a lot,
you may need to type "--" more.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2019-checkout-ambiguous-ref.sh

@@ -56,4 +56,30 @@ test_expect_success VAGUENESS_SUCCESS 'checkout reports switch to branch' '
 	test_i18ngrep ! "^HEAD is now at" stderr
 '
 
+test_expect_success 'wildcard ambiguation, paths win' '
+	git init ambi &&
+	(
+		cd ambi &&
+		echo a >a.c &&
+		git add a.c &&
+		echo b >a.c &&
+		git checkout "*.c" &&
+		echo a >expect &&
+		test_cmp expect a.c
+	)
+'
+
+test_expect_success 'wildcard ambiguation, refs lose' '
+	git init ambi2 &&
+	(
+		cd ambi2 &&
+		echo a >"*.c" &&
+		git add . &&
+		test_must_fail git show :"*.c" &&
+		git show :"*.c" -- >actual &&
+		echo a >expect &&
+		test_cmp expect actual
+	)
+'
+
 test_done