difftool: add '--no-gui' option

This commit teaches difftool to handle the '--no-gui' option. This option
negates the existing '--gui' option. The last setting given on the command
line wins.

This allows a user to configure "[alias] mdt = difftool --gui", but still
have the ability to override the setting without error:

$ git mdt --no-gui
Signed-off-by: NTim Henigan <tim.henigan@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -22,7 +22,8 @@ sub usage
 {
 	print << 'USAGE';
 usage: git difftool [-t|--tool=<tool>]
-                    [-x|--extcmd=<cmd>] [-g|--gui]
+                    [-x|--extcmd=<cmd>]
+                    [-g|--gui] [--no-gui]
                     [--prompt] [-y|--no-prompt]
                     ['git diff' options]
 USAGE
@@ -49,7 +50,7 @@ sub exe
 # parse command-line options. all unrecognized options and arguments
 # are passed through to the 'git diff' command.
 my ($difftool_cmd, $extcmd, $gui, $help, $prompt);
-GetOptions('g|gui' => \$gui,
+GetOptions('g|gui!' => \$gui,
 	'h' => \$help,
 	'prompt!' => \$prompt,
 	'y' => sub { $prompt = 0; },
@@ -75,7 +76,7 @@ sub exe
 		usage();
 	}
 }
-if (defined($gui)) {
+if ($gui) {
 	my $guitool = "";
 	$guitool = Git::config('diff.guitool');
 	if (length($guitool) > 0) {

t/t7800-difftool.sh

@@ -94,6 +94,19 @@ test_expect_success PERL 'difftool honors --gui' '
 	restore_test_defaults
 '
 
+test_expect_success PERL 'difftool --gui last setting wins' '
+	git config diff.guitool bogus-tool &&
+	git difftool --no-prompt --gui --no-gui &&
+
+	git config merge.tool bogus-tool &&
+	git config diff.tool bogus-tool &&
+	git config diff.guitool test-tool &&
+	diff=$(git difftool --no-prompt --no-gui --gui branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
 test_expect_success PERL 'difftool --gui works without configured diff.guitool' '
 	git config diff.tool test-tool &&