When creating MR from issue, make it easy to edit files

Description

When you create a merge request from an issue, it drops you into an empty MR with help for creating a file, but none for editing a file. Currently, you have to click on the branch name, then the Files menu, then select the file you want to edit. At the least, we could take you to the files list of the branch instead of the commits list. But we could go further and make a button to Edit files (if there are files to be edited). But one problem is that once you've made a single change, the no-data help goes away, and then it's back to being difficult to edit again. The branch name is still there, but it's not obvious that's where you should go. In fact, there's also a Check out branch button which semantically could mean to look at the files in the branch, but that's not what it means, so it's misleading.

Proposal

• On MR widget, link source branch name to file view instead of commit list. For consistency, the target branch name should go to the file view as well.
• In the no-data state, add a button to Edit files, which goes to the same place.
• ???