Manage ACLs for $confdir/.git

This is necessary on newer versions of Git which support Git submodules that are
_not_ located at the Git repository root directory. In practice, adding files
using "git add" in Git submodules will fail for normal users unless ACLs in
$confdir/.git are properly set.

Signed-off-by: Samuli Seppänen <samuli.seppanen@gmail.com>