[PATCH 9/9 v6] difftool: print list of valid tools with '--tool-help'

[Tim Henigan <tim.henigan@gmail.com>]

Since bc7a96a (mergetool--lib: Refactor tools into separate files,
2011-08-18), it is possible to add a new diff tool by creating a simple
script in the '$(git --exec-path)/mergetools' directory.  Updating the
difftool help text is still a manual process, and the documentation can
easily go out of sync.

Teach the command to read the list of valid tools from the 'mergetools'
directory and print them for the user when the '--tool-help' option is
given.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
---

Changes in v6:
  - Squashed "added test for difftool --tool-help" into this commit.


 Documentation/git-difftool.txt |   11 ++++++-----
 git-difftool.perl              |   16 +++++++++++++---
 t/t7800-difftool.sh            |    5 +++++
 3 files changed, 24 insertions(+), 8 deletions(-)

diff --git a/Documentation/git-difftool.txt b/Documentation/git-difftool.txt
index aba5e76..31fc2e3 100644
--- a/Documentation/git-difftool.txt
+++ b/Documentation/git-difftool.txt
@@ -36,11 +36,9 @@ OPTIONS
 
 -t <tool>::
 --tool=<tool>::
-	Use the diff tool specified by <tool>.
-	Valid diff tools are:
-	araxis, bc3, deltawalker, diffuse, emerge, ecmerge, gvimdiff,
-	kdiff3,	kompare, meld, opendiff, p4merge, tkdiff, vimdiff and
-	xxdiff.
+	Use the diff tool specified by <tool>.  Valid values include
+	emerge, kompare, meld, and vimdiff. Run `git difftool --tool-help`
+	for the list of valid <tool> settings.
 +
 If a diff tool is not specified, 'git difftool'
 will use the configuration variable `diff.tool`.  If the
@@ -68,6 +66,9 @@ of the diff post-image.  `$MERGED` is the name of the file which is
 being compared. `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$MERGED`.
 
+--tool-help::
+	Print a list of diff tools that may be used with `--tool`.
+
 -x <command>::
 --extcmd=<command>::
 	Specify a custom command for viewing diffs.
diff --git a/git-difftool.perl b/git-difftool.perl
index 0fa131c..d9be7d6 100755
--- a/git-difftool.perl
+++ b/git-difftool.perl
@@ -14,7 +14,7 @@
 use 5.008;
 use strict;
 use warnings;
-use File::Basename qw(dirname);
+use File::Basename qw(dirname basename);
 use File::Copy;
 use File::stat;
 use File::Path qw(mkpath);
@@ -28,7 +28,7 @@ sub usage
 {
 	my $exitcode = shift;
 	print << 'USAGE';
-usage: git difftool [-t|--tool=<tool>]
+usage: git difftool [-t|--tool=<tool>] [--tool-help]
                     [-x|--extcmd=<cmd>]
                     [-g|--gui] [--no-gui]
                     [--prompt] [-y|--no-prompt]
@@ -132,18 +132,28 @@ sub setup_dir_diff
 
 # parse command-line options. all unrecognized options and arguments
 # are passed through to the 'git diff' command.
-my ($difftool_cmd, $dirdiff, $extcmd, $gui, $help, $prompt);
+my ($difftool_cmd, $dirdiff, $extcmd, $gui, $help, $prompt, $tool_help);
 GetOptions('g|gui!' => \$gui,
 	'd|dir-diff' => \$dirdiff,
 	'h' => \$help,
 	'prompt!' => \$prompt,
 	'y' => sub { $prompt = 0; },
 	't|tool:s' => \$difftool_cmd,
+	'tool-help' => \$tool_help,
 	'x|extcmd:s' => \$extcmd);
 
 if (defined($help)) {
 	usage(0);
 } 
+if (defined($tool_help)) {
+	my $gitpath = Git::exec_path();
+	print "'git difftool --tool=<tool>' may be set to one of the following:\n";
+	for (glob "$gitpath/mergetools/*") {
+		next if /defaults$/;
+		print "\t" . basename($_) . "\n";
+	}
+	exit(0);
+}
 if (defined($difftool_cmd)) {
 	if (length($difftool_cmd) > 0) {
 		$ENV{GIT_DIFF_TOOL} = $difftool_cmd;
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index 478c1be..bbe71e5 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -319,6 +319,11 @@ test_expect_success PERL 'say no to the second file' '
 	echo "$diff" | stdin_doesnot_contain br2
 '
 
+test_expect_success PERL 'difftool --tool-help' '
+	tool_help=$(git difftool --tool-help) &&
+	echo "$tool_help" | stdin_contains tool
+'
+
 test_expect_success PERL 'setup change in subdirectory' '
 	git checkout master &&
 	mkdir sub &&
-- 
1.7.10.rc1.36.g15e879