hg2git.py: For the first revision, feed out full manifest

hg2git.py: For the first revision, feed out full manifest

For the mutt and hg repos, it didn't make a difference, but attempting
to run the conversion on the opensolaris repo looks like this is needed.

When we attempt to export some commit, special-case the revision number
0 and export all files the manifest has while labeling this a "full
revision export". Otherwise we do what we did before labeling this a
"delta revision export".

Signed-off-by: Rocco Rutte <pdmef@gmx.net>