grep -O: allow optional argument specifying the pager (or editor)

Suppose you want to edit all files that contain a specific search term.
Of course, you can do something totally trivial such as

	git grep -z -e <term> | xargs -0r vi +/<term>

but maybe you are happy that the same will be achieved by

	git grep -Ovi <term>

now.

[jn: rebased and added tests]

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Paolo Bonzini <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-grep.txt

@@ -14,7 +14,7 @@ SYNOPSIS
 	   [-E | --extended-regexp] [-G | --basic-regexp]
 	   [-F | --fixed-strings] [-n]
 	   [-l | --files-with-matches] [-L | --files-without-match]
-	   [-O | --open-files-in-pager]
+	   [(-O | --open-files-in-pager) [<pager>]]
 	   [-z | --null]
 	   [-c | --count] [--all-match] [-q | --quiet]
 	   [--max-depth <depth>]
@@ -105,8 +105,8 @@ OPTIONS
 	For better compatibility with 'git diff', `--name-only` is a
 	synonym for `--files-with-matches`.
 
--O::
---open-files-in-pager::
+-O [<pager>]::
+--open-files-in-pager [<pager>]::
 	Open the matching files in the pager (not the output of 'grep').
 	If the pager happens to be "less" or "vi", and the user
 	specified only one pattern, the first file is positioned at

builtin/grep.c

@@ -828,7 +828,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	int cached = 0;
 	int seen_dashdash = 0;
 	int external_grep_allowed__ignored;
-	int show_in_pager = 0;
+	const char *show_in_pager = NULL, *default_pager = "dummy";
 	struct grep_opt opt;
 	struct object_array list = { 0, 0, NULL };
 	const char **paths = NULL;
@@ -916,8 +916,9 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN(0, "all-match", &opt.all_match,
 			"show only matches from files that match all patterns"),
 		OPT_GROUP(""),
-		OPT_BOOLEAN('O', "open-files-in-pager", &show_in_pager,
-			"show matching files in the pager"),
+		{ OPTION_STRING, 'O', "open-files-in-pager", &show_in_pager,
+			"pager", "show matching files in the pager",
+			PARSE_OPT_OPTARG, NULL, (intptr_t)default_pager },
 		OPT_BOOLEAN(0, "ext-grep", &external_grep_allowed__ignored,
 			    "allow calling of grep(1) (ignored by this build)"),
 		{ OPTION_CALLBACK, 0, "help-all", &options, NULL, "show usage",
@@ -993,19 +994,16 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		argc--;
 	}
 
+	if (show_in_pager == default_pager)
+		show_in_pager = git_pager(1);
 	if (show_in_pager) {
-		const char *pager = git_pager(1);
-		if (!pager) {
-			show_in_pager = 0;
-		} else {
 		opt.name_only = 1;
 		opt.null_following_name = 1;
 		opt.output_priv = &path_list;
 		opt.output = append_path;
-			string_list_append(pager, &path_list);
+		string_list_append(show_in_pager, &path_list);
 		use_threads = 0;
 	}
-	}
 
 	if (!opt.pattern_list)
 		die("no pattern given.");

t/t7811-grep-open.sh

@@ -99,7 +99,11 @@ test_expect_success 'git grep -O jumps to line in less' '
 
 	GIT_PAGER=./less git grep -O GREP_PATTERN >out &&
 	test_cmp expect actual &&
-	test_cmp empty out
+	test_cmp empty out &&
+
+	git grep -O./less GREP_PATTERN >out2 &&
+	test_cmp expect actual &&
+	test_cmp empty out2
 '
 
 test_expect_success 'modified file' '
@@ -135,8 +139,7 @@ test_expect_success 'run from subdir' '
 		export GIT_PAGER &&
 		GIT_PAGER='\''printf "%s\n" >../args'\'' &&
 		git grep -O "enum grep_pat_token" >../out &&
-		GIT_PAGER="pwd >../dir; :" &&
-		git grep -O "enum grep_pat_token" >../out2
+		git grep -O"pwd >../dir; :" "enum grep_pat_token" >../out2
 	) &&
 	case $(cat dir) in
 	*subdir)