According to this Git commit, gitrb was creating invalid commits: git/git@daae192
Recent versions of git refuse -700, must be -0700
Fixing existing repositories is dead easy, just run every commit through a no-op environment filter and Git fixes everything itself:
git filter-branch --env-filter '' --tag-name-filter cat HEAD
This is a rebase, of course, so everyone who has cloned your repository will need to delete their local copies and clone again.