sha1_file: add repository argument to link_alt_odb_entry

sha1_file: add repository argument to link_alt_odb_entry

Add a repository argument to allow the link_alt_odb_entry caller to be
more specific about which repository to act on. This is a small
mechanical change; it doesn't change the implementation to handle
repositories other than the_repository yet.

Since the implementation does not yet work with other repositories,
use a wrapper macro to enforce that the caller passes in
the_repository as the first argument. It would be more appealing to
use BUILD_ASSERT_OR_ZERO to enforce this, but that doesn't work
because it requires a compile-time constant and common compilers like
gcc 4.8.4 do not consider "r == the_repository" a compile-time
constant.

This and the following three patches add repository arguments to
link_alt_odb_entry, read_info_alternates, link_alt_odb_entries
and prepare_alt_odb. Three out of the four functions are found
in a recursive call chain, calling each other, and one of them
accesses the repositories `objectdir` (which was migrated; it
was an obvious choice) and `ignore_env` (which we need to keep in
the repository struct for clarify); hence we will pass through the
repository unlike just the object store object + the ignore_env flag.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>