Fix core.sharedRepository = 2

Fix core.sharedRepository = 2

For compatibility reasons, "git init --shared=all" does not write
"all" into the config, but a number.  In the shared setup, you
really have to support even older clients on the _same_ repository.

But git_config_perm() did not pick up on it.

Also, "git update-server-info" failed to pick up on the shared
permissions.

This patch fixes both issues, and adds a test to prove it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: martin f krafft <madduck@madduck.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>