mail ft: disable line numbers, fold columns
au: signify on BufEnter

This resolves a number of annoyances in that, e.g., after finishing a commit
via fugitive this ensures that the signs reflect accurately the state of the
file vis-a-vis the index.

Note that it does cause a save to happen; any risk here should be mitigated
by 1) hello, git and 2) that everything is being tracked via git-wip anyways
:)