difftool: allow running outside Git worktrees with --no-index

As far as this developer can tell, the conversion from a Perl script to
a built-in caused the regression in the difftool that it no longer runs
outside of a Git worktree (with `--no-index`, of course).

It is a bit embarrassing that it took over two years after retiring the
Perl version to discover this regression, but at least we now know, and
can do something, about it.

This fixes https://github.com/git-for-windows/git/issues/2123

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/difftool.c

@@ -690,7 +690,7 @@ static int run_file_diff(int prompt, const char *prefix,
 int cmd_difftool(int argc, const char **argv, const char *prefix)
 {
 	int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
-	    tool_help = 0;
+	    tool_help = 0, no_index = 0;
 	static char *difftool_cmd = NULL, *extcmd = NULL;
 	struct option builtin_difftool_options[] = {
 		OPT_BOOL('g', "gui", &use_gui_tool,
@@ -714,6 +714,7 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 			    "tool returns a non - zero exit code")),
 		OPT_STRING('x', "extcmd", &extcmd, N_("command"),
 			   N_("specify a custom command for viewing diffs")),
+		OPT_ARGUMENT("no-index", &no_index, N_("passed to `diff`")),
 		OPT_END()
 	};
 
@@ -727,8 +728,14 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	if (tool_help)
 		return print_tool_help();
 
-	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
-	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
+	if (!no_index && !startup_info->have_repository)
+		die(_("difftool requires worktree or --no-index"));
+
+	if (!no_index){
+		setup_work_tree();
+		setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
+		setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
+	}
 
 	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
 		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);

git.c

@@ -491,7 +491,7 @@ static struct cmd_struct commands[] = {
 	{ "diff-files", cmd_diff_files, RUN_SETUP | NEED_WORK_TREE | NO_PARSEOPT },
 	{ "diff-index", cmd_diff_index, RUN_SETUP | NO_PARSEOPT },
 	{ "diff-tree", cmd_diff_tree, RUN_SETUP | NO_PARSEOPT },
-	{ "difftool", cmd_difftool, RUN_SETUP | NEED_WORK_TREE },
+	{ "difftool", cmd_difftool, RUN_SETUP_GENTLY },
 	{ "fast-export", cmd_fast_export, RUN_SETUP },
 	{ "fetch", cmd_fetch, RUN_SETUP },
 	{ "fetch-pack", cmd_fetch_pack, RUN_SETUP | NO_PARSEOPT },

t/t7800-difftool.sh

@@ -705,4 +705,14 @@ test_expect_success SYMLINKS 'difftool --dir-diff handles modified symlinks' '
 	test_cmp expect actual
 '
 
+test_expect_success 'outside worktree' '
+	echo 1 >1 &&
+	echo 2 >2 &&
+	test_expect_code 1 nongit git \
+		-c diff.tool=echo -c difftool.echo.cmd="echo \$LOCAL \$REMOTE" \
+		difftool --no-prompt --no-index ../1 ../2 >actual &&
+	echo "../1 ../2" >expect &&
+	test_cmp expect actual
+'
+
 test_done