The current rules for discarding submodule changes is that no other changed item
must be also selected. There are some bugs with the current implementation when
submodules are in folders.
For example, selecting and discarding a folder with only a nested submodule
change will currently do nothing. The submodule changes should be discarded. The
folder only contains submodule changes so it should be no different than
pressing discard on the submodule entry itself.
Also, I noticed range selecting both the folder and the submodule and then
pressing discard would be incorrectly disallowed.
As part of this, you must now press enter on a merge conflict file
to focus the merge view; you can no longer press space and if you do
it will raise an error.
We've been sometimes using lo and sometimes using my slices package, and we need to pick one
for consistency. Lo is more extensive and better maintained so we're going with that.
My slices package was a superset of go's own slices package so in some places I've just used
the official one (the methods were just wrappers anyway).
I've also moved the remaining methods into the utils package.
