sha1_file.c: cleanup hdr usage

sha1_file.c: cleanup hdr usage

Let's have hdr be a simple char pointer/array when possible, and let's
reduce its storage to 32 bytes.  Especially for sha1_loose_object_info()
where 128 bytes is way excessive and wastes extra CPU cycles inflating.

The object type is already restricted to 10 bytes in parse_sha1_header()
and the size, even if it is 64 bits, will fit in 20 decimal numbers.  So
32 bytes is plenty.

Signed-off-by: Nicolas Pitre <redacted>
Signed-off-by: Junio C Hamano <redacted>