Made the edit box only retain its internal undo history for changes made since...

Made the edit box only retain its internal undo history for changes made since the last commit or line switch rather than every state it has had since the program was started, fixing a gradual memory leak.

Originally committed to SVN as r3097.

aegisub/src/subs_edit_box.cpp

@@ -304,6 +304,8 @@ void SubsEditBox::Update (bool timeOnly,bool weak) {
 			// Video
 			VideoContext::Get()->curLine = curdiag;
 			VideoContext::Get()->UpdateDisplays(false);
+
+			TextEdit->EmptyUndoBuffer();
 		}
 		else enabled = false;
 	}
@@ -375,6 +377,8 @@ void SubsEditBox::SetToLine(int n,bool weak) {
 			if (cur) VideoContext::Get()->JumpToFrame(VFR_Output.GetFrameAtTime(cur->Start.GetMS(),true));
 		}
 	}
+
+	TextEdit->EmptyUndoBuffer();
 }