Don't die if a tag ref doesn't map to a git commit. Fixes issue #198.

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).