handle deleted files in merge conflicts

[dlyongemallo]

Right now, a file being deleted in either side is treated differently than an empty file. The ability to choose an entire side was added in #276. Implement also file creation/deletion when merging so that choosing a side when one side is deleted works correctly.

Spun off from #263.

[dlyongemallo]

Just a note that this turns out to be really hard. There are all sorts of edge cases to handle when it comes to file deletion/restoration.

[dlyongemallo]

@pksunkara What's your workflow requirements when it comes to deleted files? It turns out that this is really hard. Creating and deleting files involve OS operations and have weird edge cases (symbolic links) and side effects (triggers autocmds). It's much more involved than simply restoring or deleting a buffer's contents.

[pksunkara]

There are many scenarios here:

OURS = deleted, THEIRS = modified

1. I want to still delete the file completely ignoring any of THEIRS that's modified
2. I want to see (& resolve) the conflicts of THEIRS with another file as OURS (generally when we rename and modify a file so much that it shows up as deleted in the commit)
3. I want to copy the added changes in THEIRS so that I can add it to another file. (generally, when a new function is added to a module that's deleted). I am not sure if this usage can be collapsed into the previous one or not.

The same but reversed for when OURS = modified, THEIRS = deleted