Ignore Vim undo files
Remove vim-fugitive submodule

I wasn't really using it, and it behaves as though
GIT_DISCOVERY_ACROSS_FILESYSTEM is set, which means it tries to stat
some directories that take a long time to browse, such as /home. That
means if I run vim or view in a non-Git folder, the whole process hangs
indefinitely.
Minor prompt tweak for non-printing characters

We're supposed to use \[ and \] to wrap parts that don't have any
printable characters. Although $screen could be empty, that's not what
\[ and \] are really for.