_lp_shorten_path: set value for locals on separate line (zsh)

zsh doesn't like if we set the value of a local while we declare it.
_lp_shorten_path: don't use a sub shell anymore

Instead of using a subshell, _lp_shorten_path now directly sets its
result into the LP_PWD variable.
This is not just for style, but for speed.
Merge remote-tracking branch 'upstream/develop' into develop

Conflicts:
	liquidprompt