gitcfg: better defaults for cola.terminal

Detect if gnome-terminal, xfce4-terminal, or konsole are installed.
Prefer them over xterm when `cola.terminal` is unset.

Closes #354

Reported-by: Dave Daynard <nardholio@gmail.com>
Suggested-by: Kevin Kofler <Kevin@tigcc.ticalc.org>
Signed-off-by: David Aguilar <davvid@gmail.com>