Add abbrev_commit (first 7 chars of commit)

[yarikoptic]

In light of #39 and no need really to store full commit in the path.

BTW -- do we have access to other commit information (e.g. the datetime?) if we do, then having commit_datetime or alike would also be handy -- this would allow to have them in some sortable order within a PR or even a branch.

[jwodder]

@yarikoptic Aside from the commit hash, the commit information that all three CI systems provide (that tinuous doesn't already expose) are the commit message and the commit timestamp.