Add option to create merge request when editing/creating a file
Internal discussion: https://dev.gitlab.org/gitlab/gitlabhq/issues/2518
Currently, the new file / edit file interfaces allow you to work in the current branch or pick a new branch to commit to, but in the latter case you have to then commit, manually find the "Create Merge Request" button, pick the branch etc.
We want to go directly from the New/Edit File page to the Create Merge Request page, with the source branch set to the newly created branch and the target branch set to the branch the user was originally on.