Handle undo in `branch-add' LFUN

Backport of 2a99afee94.

src/Buffer.cpp

@@ -2328,6 +2328,7 @@ void Buffer::dispatch(FuncRequest const & func, DispatchResult & dr)
 					msg += ("\n");
 				msg += bformat(_("Branch \"%1$s\" already exists."), branch_name);
 			} else {
+				undo().recordUndoFullDocument(DocIterator());
 				branch_list.add(branch_name);
 				branch = branch_list.find(branch_name);
 				string const x11hexname = X11hexname(branch->color());

status.20x

@@ -59,6 +59,8 @@ What's new
 
 - Fix the missing menu entry for "Insert Branch <branch name>".
 
+- Handle undo in `branch-add' function.
+
 * DOCUMENTATION AND LOCALIZATION