Fallback to AppData if XDG_CONFIG_HOME is unset

In order to be a better Windows citizenship, Git should
save its configuration files on AppData folder. This can
enables git configuration files be replicated between machines
using the same Microsoft account logon which would reduce the
friction of setting up Git on new systems. Therefore, if
%APPDATA%\Git\config exists, we use it; otherwise
$HOME/.config/git/config is used.

Signed-off-by: Ariel Lourenco <ariellourenco@users.noreply.github.com>

path.c

@@ -1484,6 +1484,7 @@ int looks_like_command_line_option(const char *str)
 char *xdg_config_home_for(const char *subdir, const char *filename)
 {
 	const char *home, *config_home;
+	char *home_config = NULL;
 
 	assert(subdir);
 	assert(filename);
@@ -1492,10 +1493,26 @@ char *xdg_config_home_for(const char *subdir, const char *filename)
 		return mkpathdup("%s/%s/%s", config_home, subdir, filename);
 
 	home = getenv("HOME");
-	if (home)
-		return mkpathdup("%s/.config/%s/%s", home, subdir, filename);
+	if (home && *home)
+		home_config = mkpathdup("%s/.config/%s/%s", home, subdir, filename);
+
+	#ifdef WIN32
+	{
+		const char *appdata = getenv("APPDATA");
+		if (appdata && *appdata) {
+			char *appdata_config = mkpathdup("%s/Git/%s", appdata, filename);
+			if (file_exists(appdata_config)) {
+				if (home_config && file_exists(home_config))
+					warning("'%s' was ignored because '%s' exists.", home_config, appdata_config);
+				free(home_config);
+				return appdata_config;
+			}
+			free(appdata_config);
+		}
+	}
+	#endif
 
-	return NULL;
+	return home_config;
 }
 
 char *xdg_config_home(const char *filename)