Log sub repo context

[michaelsauter]

It would be nice to log the context object for subrepos in the start task (similar to how it is done for the main repo, see https://github.com/opendevstack/ods-pipeline/blob/master/cmd/start/main.go#L123). This would write e.g. the commit sha of the sub repo to the log, which can assist in debugging.

Right now this info can be retrieved from the written artefact manifest, but this is a bit cumbersome.