Support gitlinks in fast-import.

Currently fast-import/export cannot be used for
repositories with submodules. This patch extends
the relevant programs to make them correctly
process gitlinks.

Links can be represented by two forms of the
Modify command:

M 160000 SHA1 some/path

which sets the link target explicitly, or

M 160000 :mark some/path

where the mark refers to a commit. The latter
form can be used by importing tools to build
all submodules simultaneously in one physical
repository, and then simply fetch them apart.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>