Not sure if this is entirely the correct fix — I don't know where that ref is coming from — but not bombing out sure works better for me.
This is reproducible in a clone of http://hg.mozilla.org/mozilla-central , if you're willing to wait about a week for hg-git to finish running (and have about 14GB of RAM).
Don't die if a tag ref doesn't map to a git commit. Fixes issue #198.
It would be nice to include the tag in the warning.
I also get this problem with git://github.com/cython/cython.git itself.
Oops, I meant I get the problem from git://github.com/schacon/hg-git.git.
Dropping 2-year-old request.