Select appropriate split for vim pane in tmux

Depending on the width of the terminal it'd sometimes be better to do a
vertical split.
Currently this is based on 126 columns for vim, and a minimum of 60
columns for the terminal, although the usability of the latter is highly
dependent upon the size of the prompt.
Use minimal bash prompt

Bash prompt no longer displays hostname, and collapses the path to
display only one character per directory, except the deepest.