Import branch 'git-p4' of git://repo.or.cz/fast-export

Simon has asked that the git.git project include the git-p4 project
as at least a contrib/fast-import within git.git.  I think it makes
a lot of sense, as git-p4 nicely complements the only other in-tree
fast-import user: import-tars.perl.

git-p4 is offered under the MIT license by its authors.