Add support for "import" helper command

Add support for "import" helper command

This command, supported if the "import" capability is advertized,
allows a helper to support fetching by outputting a git-fast-import
stream.

If both "fetch" and "import" are advertized, git itself will use
"fetch" (although other users may use "import" in this case).

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Sverre Rabbelier <srabbelier@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>