Extended attributes to commits.

Commit objects now can have extended attributes using '-A' or '--attr'
flat in git-commit command. For example, you can add a 'risk' attribute
to a commit indicating the risk of the introduced changes:

git commit -m "A risky change." --attr risk=high

Next steps: show this attributes in git-log command. No you can only
view this attributes using 'git cat-file commit <commit-id>'.