Add mappings for finding conflict markers

I think these markers are common for most systems. I use them for
perforce merges. It just creates a search and there's no effort to
restore your last search because jumping to the next one with `n` is
easier than `]C`.
Add update and undo mappings

Add mappings to quickly update the diff (with an error if that doesn't
make sense for when I forget) and to undo something in the other window
(which could be useful outside of diff -- but not much).