add README and git resource
implement check script
git check reports latest ref if not specified

also always reports the resulting branch
git /in now emits fetched source
git: silence /in in /check
pass input branch through to output source

git rev-parse --abbrev-ref HEAD only works when not on a detached head. if you
are, it just says "HEAD".
fix branch in /in source output

previously a null branch would end up as "null" (a string). instead, pass it
directly through.
add git out script; can push to a branch
explicit branch promotion
use public key if specified in source
include private key in output sources
redirect all stdout to stderr, except for result
quieter output now that stderr goes to the user
silence ssh interaction
really silence them this time
new resource api
fix private key & output params
use private key in output params
add metadata
dedup author/committer metadata