Double slash when loading mu-plugin textdomain

[nsundberg]

load_muplugin_textdomain() creates double slashes for load_textdomain(). The function first uses trailingslashit() and then concatenating the path and filename with an additional /.

See lines 756-758 trunk/src/wp-includes/l10n.php#L756​:

$path = trailingslashit( WPMU_PLUGIN_DIR . '/' . ltrim( $mu_plugin_rel_path, '/' ) );

return load_textdomain( $domain, $path . '/' . $mofile );

[SergeyBiryukov]

Introduced in [37414].

I think we can keep the slash and remove the trailingslashit() call instead, for consistency with load_plugin_textdomain() and load_theme_textdomain().

[SergeyBiryukov]

In 40362:

I18N: Remove an extra slash between .mo file path and name in load_muplugin_textdomain().

Props nsundberg.
Fixes #39168.