Update merge request text to reflect editability of source/target branch

Screenshot 1 (source branch deleted):

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/81dfbc4c790a1bb047d60cd4699bb71f/image.png)

Screenshot 2 (target branch deleted):

![image](https://gitlab.com/stanhu/gitlab-ce/uploads/acec96c12d40175301ce4d4405137348/image.png)

Closes #2134

See merge request !1085