fs/Charset: default filesystem charset is UTF-8

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.

src/fs/Charset.cxx

@@ -81,6 +81,9 @@ PathToUTF8(const char *path_fs)
 {
 	assert(path_fs != nullptr);
 
+	if (fs_charset.empty())
+		return std::string(path_fs);
+
 	GIConv conv = g_iconv_open("utf-8", fs_charset.c_str());
 	if (conv == reinterpret_cast<GIConv>(-1))
 		return std::string();
@@ -108,6 +111,9 @@ PathFromUTF8(const char *path_utf8)
 {
 	assert(path_utf8 != nullptr);
 
+	if (fs_charset.empty())
+		return g_strdup(path_utf8);
+
 	return g_convert(path_utf8, -1,
 			 fs_charset.c_str(), "utf-8",
 			 nullptr, nullptr, nullptr);

src/fs/Config.cxx

@@ -60,11 +60,6 @@ ConfigureFS()
 #endif
 	}
 
-	if (charset) {
+	if (charset != nullptr)
 		SetFSCharset(charset);
-	} else {
-		LogDebug(path_domain,
-			 "setting filesystem charset to ISO-8859-1");
-		SetFSCharset("ISO-8859-1");
-	}
 }