Ignore git submodules in fast-import stream

There is no way to map them to mercurial, but this way one retains at
least some interoperability.

An alternative approach would be to export these as (fake) files or fake symlinks or so. This would allow hg users to at least remove submodule files. But on the other hand it could cause all kinds of complications if abused. Anyway, the patch as-is works for my use cases so far.

Fixes #22