Make 'cvs ci' lockless in git-cvsserver by using git-update-ref

Make 'cvs ci' lockless in git-cvsserver by using git-update-ref

This makes "ci" codepath lockless by following the usual
"remember the tip, do your thing, then compare and swap at the
end" update pattern using update-ref.  Incidentally, by updating
the code that reads where the tip of the head is to use
show-ref, it makes it safe to use in a repository whose refs are
pack-pruned.

I noticed that other parts of the program are not yet pack-refs
safe, but tried to keep the changes to the minimum.

Signed-off-by: Junio C Hamano <junkio@cox.net>