difftool: fallback on merge.guitool

In git-difftool.txt, it says

	'git difftool' falls back to 'git mergetool' config variables when the
	difftool equivalents have not been defined.

However, when `diff.guitool` is missing, it doesn't fallback to
anything. Make git-difftool fallback to `merge.guitool` when `diff.guitool` is
missing.

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

Documentation/git-difftool.txt

@@ -90,7 +90,9 @@ instead.  `--no-symlinks` is the default on Windows.
 	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`. The `--no-gui`
-	option can be used to override this setting.
+	option can be used to override this setting. If `diff.guitool`
+	is not set, we will fallback in the order of `merge.guitool`,
+	`diff.tool`, `merge.tool` until a tool is found.
 
 --[no-]trust-exit-code::
 	'git-difftool' invokes a diff tool individually on each file.

builtin/difftool.c

@@ -23,7 +23,6 @@
 #include "object-store.h"
 #include "dir.h"
 
-static char *diff_gui_tool;
 static int trust_exit_code;
 
 static const char *const builtin_difftool_usage[] = {
@@ -33,11 +32,6 @@ static const char *const builtin_difftool_usage[] = {
 
 static int difftool_config(const char *var, const char *value, void *cb)
 {
-	if (!strcmp(var, "diff.guitool")) {
-		diff_gui_tool = xstrdup(value);
-		return 0;
-	}
-
 	if (!strcmp(var, "difftool.trustexitcode")) {
 		trust_exit_code = git_config_bool(var, value);
 		return 0;
@@ -733,8 +727,8 @@ int cmd_difftool(int argc, const char **argv, const char *prefix)
 	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);
+	if (use_gui_tool)
+		setenv("GIT_MERGETOOL_GUI", "true", 1);
 	else if (difftool_cmd) {
 		if (*difftool_cmd)
 			setenv("GIT_DIFF_TOOL", difftool_cmd, 1);

t/t7800-difftool.sh

@@ -279,11 +279,27 @@ test_expect_success 'difftool + mergetool config variables' '
 	echo branch >expect &&
 	git difftool --no-prompt branch >actual &&
 	test_cmp expect actual &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
 
 	# set merge.tool to something bogus, diff.tool to test-tool
 	test_config merge.tool bogus-tool &&
 	test_config diff.tool test-tool &&
 	git difftool --no-prompt branch >actual &&
+	test_cmp expect actual &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
+
+	# set merge.tool, diff.tool to something bogus, merge.guitool to test-tool
+	test_config diff.tool bogus-tool &&
+	test_config merge.guitool test-tool &&
+	git difftool --gui --no-prompt branch >actual &&
+	test_cmp expect actual &&
+
+	# set merge.tool, diff.tool, merge.guitool to something bogus, diff.guitool to test-tool
+	test_config merge.guitool bogus-tool &&
+	test_config diff.guitool test-tool &&
+	git difftool --gui --no-prompt branch >actual &&
 	test_cmp expect actual
 '