Query remote git repo and display last commit sha.
usage: app -r ... -b ... [--service ...] [--help]
-r required repo name
-b required branch name
--service service name, for now only "github" is supported
--help display this help
./app -r laravel-shift/laravel-5.3 -b master
./app -r atlassian/bitbucketjs -b master --service=bitbucked
./app --help