Use PWD variable rather than calling pwd in cd hooks

[LinuxMercedes]

This should significantly speed up the execution of the cd hooks; for stats, see https://github.com/euank/pazi/pull/51#issuecomment-361280817

[suzaku]

Thanks for your information, I've learned from your PR and improve my project.

[jtanguy]

+1 Also, calling pwd with fish seems to output paths with ~ when inside $HOME, whereas $PWD has the ~ converted to its corresponding path