project: store objects in project-objects directly

In order to stop sharing objects/ directly between shared projects,
we have to fetch the remote objects into project-objects/ manually.
So instead of running git operations in the individual project dirs
and relying on .git/objects being symlinked to project-objects/,
tell git to store any objects it fetches in project-objects/.

We do this by leveraging the GIT_OBJECT_DIRECTORY override.  This
has been in git forever, or at least since v1.7.2 which is what we
already hard require.  This tells git to save new objects to the
specified path no matter where it's being run otherwise.

We still otherwise run git in the project-specific dir so that it
can find the right set of refs that it wants to compare against,
including local refs.  For that reason, we also have to leverage
GIT_ALTERNATE_OBJECT_DIRECTORIES to tell git where to find objects
that are not in the upstream remote.  This way git doesn't blow up
when it can't find objects only associated with local commits.

As it stands right now, the practical result is the same: since we
symlink the project objects/ dir to the project-objects/ tree, the
default objects dir, the one we set $GIT_OBJECT_DIRECTORY to, and
the one we set $GIT_ALTERNATE_OBJECT_DIRECTORIES to are actually
all the same.  So this commit by itself should be safe.  But in a
follow up commit, we can replace the symlink with a separate dir
and git will keep working.

Bug: https://crbug.com/gerrit/15553
Change-Id: Ie4e654aec3e1ee307eee925a54908a2db6a5869f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/328100
Reviewed-by: Jack Neus <jackneus@google.com>
Tested-by: Mike Frysinger <vapier@google.com>

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