sha1-file: split OBJECT_INFO_FOR_PREFETCH

sha1-file: split OBJECT_INFO_FOR_PREFETCH

The OBJECT_INFO_FOR_PREFETCH bitflag was added to sha1-file.c in 0f4a4fb1
(sha1-file: support OBJECT_INFO_FOR_PREFETCH, 2019-03-29) and is used to
prevent the fetch_objects() method when enabled.

However, there is a problem with the current use. The definition of
OBJECT_INFO_FOR_PREFETCH is given by adding 32 to OBJECT_INFO_QUICK. This is
clearly stated above the definition (in a comment) that this is so
OBJECT_INFO_FOR_PREFETCH implies OBJECT_INFO_QUICK. The problem is that using
"flag & OBJECT_INFO_FOR_PREFETCH" means that OBJECT_INFO_QUICK also implies
OBJECT_INFO_FOR_PREFETCH.

Split out the single bit from OBJECT_INFO_FOR_PREFETCH into a new
OBJECT_INFO_SKIP_FETCH_OBJECT as the single bit and keep
OBJECT_INFO_FOR_PREFETCH as the union of two flags. This allows a clearer use
of flag checking while also keeping the implication of OBJECT_INFO_QUICK.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>