git needs to depend on dev-perl/DBI because git-cvsserver doesn't work without it.
Maybe you could re-read the einfo that pkg_postinst provides?
Realistically, I guess it should be a USE option then, rather than installing broken tools. You see, I may have imported git quite some time ago, and I've only needed git-cvsserver functionality now, which wouldn't naturally make me go off and read the pkg_postinst stuff.