save git sha1 in started.json

[ixdy]

The git "version" is really just git describe --tags slightly munged, which omits the git sha1 if the tree is checked out at a tag (e.g. for a release). Our tooling often wants a git sha1, however, so we should probably save it explicitly in a separate field, rather than trying to parse it from the version.

Conveniently the git sha1 is embedded in kubernetes binaries, along with the version; less conveniently, this means we have to parse kubectl version output.