sha1-file: split OBJECT_INFO_FOR_PREFETCH

[derrickstolee]

The OBJECT_INFO_FOR_PREFETCH bitflag was added to sha1-file.c in 0f4a4fb
(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.

@dscho: this might fix the problems with the VFS for Git functional tests.

[dscho]

Good catch!

Maybe you can run the functional tests without this (but with my backwards-compatibility hack to support post-index-changed, still), to see whether the functional tests fail, and then re-run with this PR merged?

[derrickstolee]

@dscho I already updated microsoft/VFSForGit#1209 to use the new "post-index-change" name and got the tests to pass except the one that this should fix. I'll try creating an installer on this PR and update that PR accordingly.

[dscho]

I already updated microsoft/VFSForGit#1209 to use the new "post-index-change" name and got the tests to pass except the one that this should fix.

Never mind, then. Let's merge this here PR?

[derrickstolee]

@dscho it looks like this PR didn't improve the situation, but the patch is being taken upstream rather quickly. No need to merge this PR.