Allow resolving merge conflicts where one side deleted the file

If I try to merge branches a and b, and branch a edited file foo while branch b deleted it, we don't allow resolving the conflicts in the UI. We could, we just need to figure out a way to display this.

Here's an example of what the CLI does:

$ git merge conflict-start
CONFLICT (modify/delete): files/ruby/version_info.rb deleted in conflict-start and modified in HEAD. Version HEAD of files/ruby/version_info.rb left in tree.
Automatic merge failed; fix conflicts and then commit the result.
$ git status
On branch conflict-missing-side
You have unmerged paths.
  (fix conflicts and run "git commit")

Changes to be committed:

	modified:   CHANGELOG
	modified:   files/images/logo-white.png
	modified:   files/markdown/ruby-style-guide.md
	modified:   files/ruby/popen.rb
	modified:   files/ruby/regex.rb

Unmerged paths:
  (use "git add/rm <file>..." as appropriate to mark resolution)

	deleted by them: files/ruby/version_info.rb

In this example, files/ruby/version_info.rb has the content from the branch which edited it, but isn't in the index because the other side of the merge deleted it.