grep: stop looking at random places for .gitattributes

grep searches for .gitattributes using "name" field in struct
grep_source but that field is not real on-disk path name. For example,
"grep pattern rev" fills the field with "rev:path", and Git looks for
.gitattributes in the (non-existent but exploitable) path "rev:path"
instead of "path".

This patch passes real paths down to grep_source_load_driver() when:

 - grep on work tree
 - grep on the index
 - grep a commit (or a tag if it points to a commit)

so that these cases look up .gitattributes at proper paths.
.gitattributes lookup is disabled in all other cases.

Initial-work-by: Jeff King <peff@peff.net>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7008-grep-binary.sh

@@ -111,6 +111,28 @@ test_expect_success 'grep respects binary diff attribute' '
 	test_cmp expect actual
 '
 
+test_expect_success 'grep --cached respects binary diff attribute' '
+	git grep --cached text t >actual &&
+	test_cmp expect actual
+'
+
+test_expect_success 'grep --cached respects binary diff attribute (2)' '
+	git add .gitattributes &&
+	rm .gitattributes &&
+	git grep --cached text t >actual &&
+	test_when_finished "git rm --cached .gitattributes" &&
+	test_when_finished "git checkout .gitattributes" &&
+	test_cmp expect actual
+'
+
+test_expect_success 'grep revision respects binary diff attribute' '
+	git commit -m new &&
+	echo "Binary file HEAD:t matches" >expect &&
+	git grep text HEAD -- t >actual &&
+	test_when_finished "git reset HEAD^" &&
+	test_cmp expect actual
+'
+
 test_expect_success 'grep respects not-binary diff attribute' '
 	echo binQary | q_to_nul >b &&
 	git add b &&