Set minimum size for "Version Control" diff bottom panel

Allows to see version diffs without having to expand the bottom panel
manually when clicking on a changed file in the "Commit" dock.

(cherry picked from commit 0fce7aea882f4e5fa9f177a66f62bca2a7bb2127)

editor/plugins/version_control_editor_plugin.cpp

@@ -489,6 +489,7 @@ VersionControlEditorPlugin::VersionControlEditorPlugin() {
 
 	version_control_dock = memnew(PanelContainer);
 	version_control_dock->set_v_size_flags(Control::SIZE_EXPAND_FILL);
+	version_control_dock->set_custom_minimum_size(Size2(0, 300) * EDSCALE);
 	version_control_dock->hide();
 
 	diff_vbc = memnew(VBoxContainer);