difftool: Fix '--gui' when diff.guitool is unconfigured

When diff.guitool is unconfigured and "--gui" is specified
git-difftool dies with the following error message:

	config diff.guitool: command returned error: 1

Catch the error so that the "--gui" flag is a no-op when
diff.guitool is unconfigured.

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool.perl

@@ -78,11 +78,13 @@ sub generate_command
 			next;
 		}
 		if ($arg eq '-g' || $arg eq '--gui') {
-			my $tool = Git::command_oneline('config',
-			                                'diff.guitool');
-			if (length($tool)) {
-				$ENV{GIT_DIFF_TOOL} = $tool;
-			}
+			eval {
+				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') {

t/t7800-difftool.sh

@@ -92,6 +92,15 @@ test_expect_success 'difftool honors --gui' '
 	restore_test_defaults
 '
 
+test_expect_success 'difftool --gui works without configured diff.guitool' '
+	git config diff.tool 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