safe.directory: normalize the configured path

safe.directory: normalize the configured path

The pathname of a repository comes from getcwd() and it could be a
path aliased via symbolic links, e.g., the real directory may be
/home/u/repository but a symbolic link /home/u/repo may point at it,
and the clone request may come as "git clone file:///home/u/repo/"

A request to check if /home/u/repository is safe would be rejected
if the safe.directory configuration allows /home/u/repo/ but not its
alias /home/u/repository/.  Normalize the paths configured for the
safe.directory configuration variable before comparing them with the
path being checked.

Two and a half things to note, compared to the previous step to
normalize the actual path of the suspected repository, are:

 - A configured safe.directory may be coming from .gitignore in the
   home directory that may be shared across machines.  The path
   meant to match with an entry may not necessarily exist on all of
   such machines, so not being able to convert them to real path on
   this machine is *not* a condition that is worthy of warning.
   Hence, we ignore a path that cannot be converted to a real path.

 - A configured safe.directory is essentially a random string that
   user throws at us, written completely unrelated to the directory
   the current process happens to be in.  Hence it makes little
   sense to give a non-absolute path.  Hence we ignore any
   non-absolute paths, except for ".".

 - The safe.directory set to "." was once advertised on the list as
   a valid workaround for the regression caused by the overly tight
   safe.directory check introduced in 2.45.1; we treat it to mean
   "if we are at the top level of a repository, it is OK".
   (cf. <834862fd-b579-438a-b9b3-5246bf27ce8a@gmail.com>).

Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>