init --shared=0666 isn't

Using git 1.7.0.2 on Mac OS X 10.6.2:

$ git init --bare --shared=0666 /tmp/shared.git
$ git --git-dir=/tmp/shared.git remote add --mirror \
        origin git://git.kernel.org/pub/scm/git/git.git
$ git --git-dir=/tmp/shared.git fetch origin

# login as someone else; different uid, different gid, then:

$ git --git-dir=/tmp/shared.git fetch origin

error: cannot open /tmp/shared.git/FETCH_HEAD: Permission denied

FETCH_HEAD is owned by the original user, with 0644 permissions,
 not 0666 as originally requested.

I've only had a quick glance at the source - my first guess
 is that in builtin-fetch.c, store_updated_refs and 
truncate_fetch_head should call adjust_shared_perm after
closing the file being written? Or, should an empty FETCH_HEAD
with appropriate shared perms be written during init?

In my case I'm using /tmp/shared.git as a mirror for other
 local repositories init'ed with --reference=/path/to/mirror.git
and all I'll be doing if fetching into it periodically, so I think fixing 
FETCH_HEAD will suffice for me. 

But, I wonder if there are other files written that will need fixing also - ORIG_HEAD, MERGE_HEAD?

Is there a workaround for this or am I doing something wrong? Thanks for any help.

Regards, Steve