grep: do not unnecessarily query repo for "--"

When running a command of the form

  git grep --no-index pattern -- path

in the absence of a Git repository, an error message will be printed:

  fatal: BUG: setup_git_env called without repository

This is because "git grep" tries to interpret "--" as a rev. "git grep"
has always tried to first interpret "--" as a rev for at least a few
years, but this issue was upgraded from a pessimization to a bug in
commit 59332d1 ("Resurrect "git grep --no-index"", 2010-02-06), which
calls get_sha1 regardless of whether --no-index was specified. This bug
appeared to be benign until commit b1ef400 ("setup_git_env: avoid blind
fall-back to ".git"", 2016-10-20) when Git was taught to die in this
situation.  (This "git grep" bug appears to be one of the bugs that
commit b1ef400 is meant to flush out.)

Therefore, always interpret "--" as signaling the end of options,
instead of trying to interpret it as a rev first.

Signed-off-by: Jonathan Tan <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jonathantanmy

builtin/grep.c

@@ -1154,6 +1154,11 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		const char *arg = argv[i];
 		unsigned char sha1[20];
 		struct object_context oc;
+		if (!strcmp(arg, "--")) {
+			i++;
+			seen_dashdash = 1;
+			break;
+		}
 		/* Is it a rev? */
 		if (!get_sha1_with_context(arg, 0, sha1, &oc)) {
 			struct object *object = parse_object_or_die(sha1, arg);
@@ -1162,10 +1167,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			add_object_array_with_path(object, arg, &list, oc.mode, oc.path);
 			continue;
 		}
-		if (!strcmp(arg, "--")) {
-			i++;
-			seen_dashdash = 1;
-		}
 		break;
 	}
 

t/t7810-grep.sh

@@ -982,6 +982,21 @@ test_expect_success 'grep -e -- -- path' '
 	test_cmp expected actual
 '
 
+test_expect_success 'grep --no-index pattern -- path' '
+	rm -fr non &&
+	mkdir -p non/git &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		echo hello >hello &&
+		echo goodbye >goodbye &&
+		echo hello:hello >expect &&
+		git grep --no-index o -- hello >actual &&
+		test_cmp expect actual
+	)
+'
+
 cat >expected <<EOF
 hello.c:int main(int argc, const char **argv)
 hello.c:	printf("Hello world.\n");