builtin/grep: add grep.fallbackToNoIndex config

Currently when git grep is used outside of a git repository without the
--no-index option git simply dies.  For convenience, add a
grep.fallbackToNoIndex configuration variable.  If set to true, git grep
behaves like git grep --no-index if it is run outside of a git
repository.  It defaults to false, preserving the current behavior.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1399,6 +1399,10 @@ grep.extendedRegexp::
 	option is ignored when the 'grep.patternType' option is set to a value
 	other than 'default'.
 
+grep.fallbackToNoIndex::
+	If set to true, fall back to git grep --no-index if git grep
+	is executed outside of a git repository.  Defaults to false.
+
 gpg.program::
 	Use this custom program instead of "gpg" found on $PATH when
 	making or verifying a PGP signature. The program must support the

Documentation/git-grep.txt

@@ -56,6 +56,10 @@ grep.extendedRegexp::
 grep.fullName::
 	If set to true, enable '--full-name' option by default.
 
+grep.fallbackToNoIndex::
+	If set to true, fall back to git grep --no-index if git grep
+	is executed outside of a git repository.  Defaults to false.
+
 
 OPTIONS
 -------

builtin/grep.c

@@ -770,9 +770,15 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			     PARSE_OPT_NO_INTERNAL_HELP);
 	grep_commit_pattern_type(pattern_type_arg, &opt);
 
-	if (use_index && !startup_info->have_repository)
-		/* die the same way as if we did it at the beginning */
-		setup_git_directory();
+	if (use_index && !startup_info->have_repository) {
+		int fallback = 0;
+		git_config_get_bool("grep.fallbacktonoindex", &fallback);
+		if (fallback)
+			use_index = 0;
+		else
+			/* die the same way as if we did it at the beginning */
+			setup_git_directory();
+	}
 
 	/*
 	 * skip a -- separator; we know it cannot be

t/t7810-grep.sh

@@ -821,6 +821,47 @@ test_expect_success 'outside of git repository' '
 	)
 '
 
+test_expect_success 'outside of git repository with fallbackToNoIndex' '
+	rm -fr non &&
+	mkdir -p non/git/sub &&
+	echo hello >non/git/file1 &&
+	echo world >non/git/sub/file2 &&
+	cat <<-\EOF >non/expect.full &&
+	file1:hello
+	sub/file2:world
+	EOF
+	echo file2:world >non/expect.sub &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git -c grep.fallbackToNoIndex=false grep o &&
+		git -c grep.fallbackToNoIndex=true grep o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+		cd sub &&
+		test_must_fail git -c grep.fallbackToNoIndex=false grep o &&
+		git -c grep.fallbackToNoIndex=true grep o >../../actual.sub &&
+		test_cmp ../../expect.sub ../../actual.sub
+	) &&
+
+	echo ".*o*" >non/git/.gitignore &&
+	(
+		GIT_CEILING_DIRECTORIES="$(pwd)/non" &&
+		export GIT_CEILING_DIRECTORIES &&
+		cd non/git &&
+		test_must_fail git -c grep.fallbackToNoIndex=false grep o &&
+		git -c grep.fallbackToNoIndex=true grep --exclude-standard o >../actual.full &&
+		test_cmp ../expect.full ../actual.full &&
+
+		{
+			echo ".gitignore:.*o*" &&
+			cat ../expect.full
+		} >../expect.with.ignored &&
+		git -c grep.fallbackToNoIndex grep --no-exclude o >../actual.full &&
+		test_cmp ../expect.with.ignored ../actual.full
+	)
+'
+
 test_expect_success 'inside git repository but with --no-index' '
 	rm -fr is &&
 	mkdir -p is/git/sub &&