Make next_commit_id a member variable rather than a static

Multiple instances of SubsController should not share it.

src/subs_controller.cpp

@@ -310,8 +310,6 @@ void SubsController::SetFileName(agi::fs::path const& path) {
 void SubsController::OnCommit(AssFileCommit c) {
 	if (c.message.empty() && !undo_stack.empty()) return;
 
-	static int next_commit_id = 1;
-
 	commit_id = next_commit_id++;
 	// Allow coalescing only if it's the last change and the file has not been
 	// saved since the last change

src/subs_controller.h

@@ -48,6 +48,10 @@ class SubsController {
 	int saved_commit_id = 0;
 	/// Last autosaved version of this file
 	int autosaved_commit_id = 0;
+	/// Version to use for the next commit
+	/// Needed to handle Save -> Undo -> Edit, which would result in the file
+	/// being marked unmodified if we reused commit IDs
+	int next_commit_id = 1;
 
 	/// Timer for triggering autosaves
 	wxTimer autosave_timer;