delta-islands: stop depending on `the_repository`

delta-islands: stop depending on `the_repository`

There are multiple sites in "delta-islands.c" where we use the
global `the_repository` variable, either explicitly or implicitly by
using `the_hash_algo`.

Refactor the code to stop using `the_repository`. In most cases this is
trivial because we already had a repository available in the calling
context, with the only exception being `propagate_island_marks()`. Adapt
it so that the repository gets passed in via a parameter.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>