"core.sharedrepository = 0mode" should set, not loosen

"core.sharedrepository = 0mode" should set, not loosen

This fixes the behaviour of octal notation to how it is defined in the
documentation, while keeping the traditional "loosen only" semantics
intact for "group" and "everybody".

Three main points of this patch are:

 - For an explicit octal notation, the internal shared_repository variable
   is set to a negative value, so that we can tell "group" (which is to
   "OR" in 0660) and 0660 (which is to "SET" to 0660);

 - git-init did not set shared_repository variable early enough to affect
   the initial creation of many files, notably copied templates and the
   configuration.  We set it very early when a command-line option
   specifies a custom value.

 - Many codepaths create files inside $GIT_DIR by various ways that all
   involve mkstemp(), and then call move_temp_to_file() to rename it to
   its final destination.  We can add adjust_shared_perm() call here; for
   the traditional "loosen-only", this would be a no-op for many codepaths
   because the mode is already loose enough, but with the new behaviour it
   makes a difference.

Signed-off-by: Junio C Hamano <gitster@pobox.com>