init-db: improve the filemode trustability check

Some file systems do not support the executable bit:

  a) The user executable bit is always 0, e.g. VFAT mounted with
     -onoexec

  b) The user executable bit is always 1, e.g. cifs mounted with
     -ofile_mode=0755

  c) There are system where user executable bit is 1 even if it
     should be 0 like b), but the file mode can be maintained
     locally. chmod -x changes the file mode from 0766 to 0666,
     until the file system is unmounted and remounted and the file
     mode is 0766 again.

     This been observed when a Windows machine with NTFS exports a share to
     Mac OS X via smb or afp.

Case a) and b) are handled by the current code.  Case c) qualifies
as "non trustable executable bit" and core.filemode should be false,
but this is currently not done.

Detect when ".git/config" has the user executable bit set after
creat(".git/config", 0666) and set core.filemode to false.  Because
the permission bits on the file is whatever the end user already had
when we are asked to reinitialise an existing repository, and do not
give any information on the filesystem behaviour, do this only when
running "git init" to create a new repository.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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