Git Discovery Reworked

[Jason A. Donenfeld]

Hi,

For pass 1.7, there's going to be better support for nested git
repositories. The PASSWORD_STORE_GIT variable has been removed, and in
its place, pass will automatically discover any git sub-directories.
The logic for this is a bit tricky, so please test. Changes just
landed in master.

Comments? Issues?

Thanks,
Jason