object-file: use designated initializers for "struct git_hash_algo"

object-file: use designated initializers for "struct git_hash_algo"

As with the preceding commit, change another file-level struct
assignment to use designated initializers.

Retain the ".name = NULL" etc. in the case of the first element of
"unknown hash algorithm", to make it explicit that we're intentionally
not setting those, it's not just that we forgot.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>