Allow difftool to be run outside of Git worktrees

It was reported in https://github.com/git-for-windows/git/issues/2123 that git difftool --no-index fails to work outside worktrees, even if it should work.

I fear this is a regression I introduced over two years ago (!) when I converted the Perl script to C.

At least now that I know about the bug, I can fix it.

Changes since v1:

* Instead of ad-hoc parsing to look for --no-index, the OPT_ARGUMENT() was enhanced and not has its first real user! After all those lonely, long years (11 years, 1 month and 11 days), what a wonderful thing to celebrate on π day.
* The test now uses the nongit helper.
* The test uses test_cmp to make diagnosing regressions easier.

Johannes Schindelin (3):
difftool: remove obsolete (and misleading) comment
parse-options: make OPT_ARGUMENT() more useful
difftool: allow running outside Git worktrees with --no-index

 Documentation/technical/api-parse-options.txt |  4 +++-
 builtin/difftool.c                            | 14 ++++++++++----
 git.c                                         |  2 +-
 parse-options.c                               |  2 ++
 parse-options.h                               |  4 ++--
 t/helper/test-parse-options.c                 |  2 +-
 t/t7800-difftool.sh                           | 10 ++++++++++
 7 files changed, 29 insertions(+), 9 deletions(-)

base-commit: e902e9bcae2010bc42648c80ab6adc6c5a16a4a5
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-163%2Fdscho%2Fdifftool-no-index-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-163/dscho/difftool-no-index-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/163

Range-diff vs v1:

 1:  2ad91f19c5 = 1:  2ad91f19c5 difftool: remove obsolete (and misleading) comment
 -:  ---------- > 2:  10775638ad parse-options: make OPT_ARGUMENT()  more useful
 2:  9f6eb60eee ! 3:  8cca9f800c difftool: allow running outside Git worktrees with --no-index
     @@ -22,24 +22,24 @@
       {
       	int use_gui_tool = 0, dir_diff = 0, prompt = -1, symlinks = 0,
      -	    tool_help = 0;
     -+	    tool_help = 0, i, no_index = 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,
     +@@
     + 			    "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()
     + 	};
     + 
      @@
       	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);
     -+	for (i = 0; i < argc; i++)
     -+		if (!strcmp(argv[i], "--"))
     -+			break;
     -+		else if (!strcmp(argv[i], "--no-index")) {
     -+			no_index = 1;
     -+			break;
     -+		}
     -+
      +	if (!no_index && !startup_info->have_repository)
      +		die(_("difftool requires worktree or --no-index"));
      +
     @@ -73,13 +73,13 @@
       '
       
      +test_expect_success 'outside worktree' '
     -+	mkdir outside &&
     -+	echo 1 >outside/1 &&
     -+	echo 2 >outside/2 &&
     -+	test_expect_code 1 env GIT_CEILING_DIRECTORIES="$(pwd)" git \
     ++	echo 1 >1 &&
     ++	echo 2 >2 &&
     ++	test_expect_code 1 nongit git \
      +		-c diff.tool=echo -c difftool.echo.cmd="echo \$LOCAL \$REMOTE" \
     -+		-C outside difftool --no-prompt --no-index 1 2 >out &&
     -+	test "1 2" = "$(cat out)"
     ++		difftool --no-prompt --no-index ../1 ../2 >actual &&
     ++	echo "../1 ../2" >expect &&
     ++	test_cmp expect actual
      +'
      +
       test_done

gitgitgadget