gcc-tarball lorry is somehow failing

See this email for more information.

Part of it describing the issue:

Firstly, the GCC 7.2 tarball from https://ftp.gnu.org/gnu/gcc/gcc-7.2.0/
doesn't import correctly with Lorry's `lorry.tar-importer` script, nor
with the `contrib/fast-import/import-tars.perl` script from Git that it
is based on.

The script fails with an error like this:

     fatal: Unsupported command: // any later version.
     fast-import: dumping crash report to fast_import_crash_10201

The exact message varies, but the issue seems to be that it's sending
source code as `git fast-import` commands, causing `git fast-import` to
raise an error.