sha1_file: make read_info_alternates static

sha1_file: make read_info_alternates static

read_info_alternates is not used from outside, so let's make it static.

We have to declare the function before link_alt_odb_entry instead of
moving the code around, link_alt_odb_entry calls read_info_alternates,
which in turn calls link_alt_odb_entry.

Signed-off-by: Stefan Beller <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>