If under Git (may be git-annex/datalad), add basic information about state as well into summary

[yarikoptic]

The idea is really to annotate the validator output with descriptor/signature of the state of the dataset which was validated, so if the output of the bids-validator is provided as supplementary information for a paper, it could identify/confirm which state it is for.

An obvious way to describe the state is when dataset is under VCS. So if detected to be under git, could provide output of git describe --tags --always or just a commit hexsha + either state is clean (an empty output of git status --porcelain could be a sign for it being clean).