tmux: Support <1.9 and >=1.9.

As of version 1.9, Tmux requires you to explictly tell it you want to retain
the current working directory when creating a new pane. Unfortunately it seems
they also changed the way 'pane_current_path' works in the same release. The
result is that ignoring the directory setting causes unexpected behaviour on
>=1.9 and setting it explicitly causes unexpected behaviour on <1.9. Not so
pleasant for poor fools like me who use machines with different versions of
Tmux. This commit makes everything hunky dory in all versions.

This also effectively reverts 67bacdbe4e2b04ecd4093089415c8c0ef241c574.