difftool: make --gui, --tool and --extcmd mutually exclusive

In git-difftool, these options specify which tool to ultimately run. As
a result, they are logically conflicting. Explicitly disallow these
options from being used together.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Denton-L

builtin/difftool.c

@@ -730,6 +730,9 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	setenv(GIT_DIR_ENVIRONMENT, absolute_path(get_git_dir()), 1);
 	setenv(GIT_WORK_TREE_ENVIRONMENT, absolute_path(get_git_work_tree()), 1);
 
+	if (use_gui_tool + !!difftool_cmd + !!extcmd > 1)
+		die(_("--gui, --tool and --extcmd are mutually exclusive"));
+
 	if (use_gui_tool && diff_gui_tool && *diff_gui_tool)
 		setenv("GIT_DIFF_TOOL", diff_gui_tool, 1);
 	else if (difftool_cmd) {

t/t7800-difftool.sh

@@ -705,4 +705,12 @@ test_expect_success SYMLINKS 'difftool --dir-diff handles modified symlinks' '
 	test_cmp expect actual
 '
 
+test_expect_success 'difftool --gui, --tool and --extcmd are exclusive' '
+	difftool_test_setup &&
+	test_must_fail git difftool --gui --tool=test-tool &&
+	test_must_fail git difftool --gui --extcmd=cat &&
+	test_must_fail git difftool --tool=test-tool --extcmd=cat &&
+	test_must_fail git difftool --gui --tool=test-tool --extcmd=cat
+'
+
 test_done