Condense commits already in target branch when updating merge request source branch.

Resolves https://dev.gitlab.org/gitlab/gitlabhq/issues/2043.

![Screen_Shot_2015-03-06_at_17.17.13](https://dev.gitlab.org/gitlab/gitlabhq/uploads/8d03540b7b7294ce07005e6f3db6ed3d/Screen_Shot_2015-03-06_at_17.17.13.png)

(Not shown in screenshot, but works fine with rebases as well)

See merge request !1648