git/export: use 'date -f' instead of 'date -m'

git/export: use 'date -f' instead of 'date -m'

The '-m' flag was added to date largely
to support git scripts. It predates the
tmdate code, which is why it exists, but
it's a recent enough addition that nothing
I'm aware of uses it, other than git.

As a result, it would be good to remove
it, so let's do that.