contrib/mtn2git: Add a tool to export the monotone database to git

contrib/mtn2git: Add a tool to export the monotone database to git
    This is just a snapshot from my mtn tree on the request of some random
    people sitting in this room...
    This will use the automate interface of monotone to export the history
    in a format readable by git-fast-import
    ../../mtn2git/mtn2git.py --db ../../../mtn2git/OE.mtn --marks=../mtn2git-marks --status=../mtn2git-status.v2 | git-fast-import --date-format=rfc2822 --import-marks=../git-import-marks --export-marks=../git-import-marks