git-difftool: Add '--gui' for selecting a GUI tool

Users might prefer to have git-difftool use a different
tool when run from a Git GUI.

This teaches git-difftool to honor 'diff.guitool' when
the '--gui' option is specified.  This allows users to
configure their preferred command-line diff tool in
'diff.tool' and a GUI diff tool in 'diff.guitool'.

Reference: http://article.gmane.org/gmane.comp.version-control.git/133386Signed-off-by: NDavid Aguilar <davvid@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-difftool.txt

@@ -58,6 +58,12 @@ is set to the name of the temporary file containing the contents
 of the diff post-image.  `$BASE` is provided for compatibility
 with custom merge tool commands and has the same value as `$LOCAL`.
 
+-g::
+--gui::
+	When 'git-difftool' is invoked with the `-g` or `--gui` option
+	the default diff tool will be read from the configured
+	`diff.guitool` variable instead of `diff.tool`.
+
 See linkgit:git-diff[1] for the full list of supported options.
 
 CONFIG VARIABLES
@@ -68,6 +74,9 @@ difftool equivalents have not been defined.
 diff.tool::
 	The default diff tool to use.
 
+diff.guitool::
+	The default diff tool to use when `--gui` is specified.
+
 difftool.<tool>.path::
 	Override the path for the given tool.  This is useful in case
 	your tool is not in the PATH.

git-difftool.perl

@@ -15,13 +15,16 @@
 use Cwd qw(abs_path);
 use File::Basename qw(dirname);
 
+require Git;
+
 my $DIR = abs_path(dirname($0));
 
 
 sub usage
 {
 	print << 'USAGE';
-usage: git difftool [--tool=<tool>] [-y|--no-prompt] ["git diff" options]
+usage: git difftool [-g|--gui] [-t|--tool=<tool>] [-y|--no-prompt]
+                    ["git diff" options]
 USAGE
 	exit 1;
 }
@@ -63,6 +66,14 @@ sub generate_command
 			$ENV{GIT_DIFF_TOOL} = substr($arg, 7);
 			next;
 		}
+		if ($arg eq '-g' || $arg eq '--gui') {
+			my $tool = Git::command_oneline('config',
+			                                'diff.guitool');
+			if (length($tool)) {
+				$ENV{GIT_DIFF_TOOL} = $tool;
+			}
+			next;
+		}
 		if ($arg eq '-y' || $arg eq '--no-prompt') {
 			$ENV{GIT_DIFFTOOL_NO_PROMPT} = 'true';
 			delete $ENV{GIT_DIFFTOOL_PROMPT};

t/t7800-difftool.sh

@@ -19,6 +19,7 @@ remove_config_vars()
 {
 	# Unset all config variables used by git-difftool
 	git config --unset diff.tool
+	git config --unset diff.guitool
 	git config --unset difftool.test-tool.cmd
 	git config --unset difftool.prompt
 	git config --unset merge.tool
@@ -77,6 +78,17 @@ test_expect_success 'difftool ignores bad --tool values' '
 	test "$diff" = ""
 '
 
+test_expect_success 'difftool honors --gui' '
+	git config merge.tool bogus-tool &&
+	git config diff.tool bogus-tool &&
+	git config diff.guitool test-tool &&
+
+	diff=$(git difftool --no-prompt --gui branch) &&
+	test "$diff" = "branch" &&
+
+	restore_test_defaults
+'
+
 # Specify the diff tool using $GIT_DIFF_TOOL
 test_expect_success 'GIT_DIFF_TOOL variable' '
 	git config --unset diff.tool