repository: repo_submodule_init to take a submodule struct

repository: repo_submodule_init to take a submodule struct

When constructing a struct repository for a submodule for some revision
of the superproject where the submodule is not contained in the index,
it may not be present in the working tree currently either. In that
situation giving a 'path' argument is not useful. Upgrade the
repo_submodule_init function to take a struct submodule instead.
The submodule struct can be obtained via submodule_from_{path, name} or
an artificial submodule struct can be passed in.

While we are at it, rename the repository struct in the repo_submodule_init
function, which is to be initialized, to a name that is not confused with
the struct submodule as easily. Perform such renames in similar functions
as well.

Also move its documentation into the header file.

Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>