My /etc directory is under version control.  Because this repository
contains confidental information, _darcs is not readable by normal
users.

Currently Darcs treats /etc as "not a repository".  It would be better
if could recognize an unreadable _darcs and report something like "is
a repository, but not accessible".

Here is a transcript demonstrating the problem.

    $ sudo sh -c 'umask 077 && darcs init'
    $ ls -al
    total 0
    drwx------ 3 twb  twb   60 2009-04-04 22:26 .
    drwxrwxrwt 9 root root 600 2009-04-04 22:25 ..
    drwx------ 5 root root 140 2009-04-04 22:26 _darcs
    $ darcs show repo

    darcs failed:  Unable to "darcs show repo" here.

    You need to be in a repository directory to run this command.

I suppose here we'd be implicitly assuming that anything with a _darcs
directory is a repository.  Will this actually catch a lot of cases? 
Wouldn't the _darcs directory also be invisible in a lot of these (-x on
the outer dir?).  Anyway, I guess it couldn't really hurt except maybe
in KISS terms.