Adds an env variable to specify a remote

I often want to look at pull requests from remotes other than origin.
Often, I want to look at "upstream" (where origin is mine, and upstream
is the primary maintainer's) or other people's forks of my thing, etc.

Usage:
GIT_REMOTE=upstream git pulls list

or set it permanently in a profile or whatever.
Merge pull request #54 from todb/feature/set-custom-remote

Adds an env variable to specify a remote