dont auto-vivify empty entries in %repos...

before this, trying to access a wild repo would create an empty hash in
%repos.  This is pretty harmless, but at some later point, memberships()
would try to use that in a pattern, attempting to match the real repo
being access-checked.

Which is still fine if your repo doesn't look like "libstdc++" AND
you're using some recent perl.

However, for perl 5.8.8, and if the repo has a ++ in it, perl barfs.

Here's a test program to check your perl:

    #!/usr/bin/perl

    $base="foo/u1/libstdc++";
    $i="foo/u1/libstdc++";

    if ( $base =~ /^$i$/ ) {
        print 1;
    } else {
        print 2;
    }

On 5.14.2 I get "2".  On 5.8.8 I get:

    Nested quantifiers in regex; marked by <-- HERE in m/^foo/u1/libstdc++ <-- HERE $/ at ./aa.pl line 6.

src/lib/Gitolite/Conf/Load.pm

@@ -255,7 +255,7 @@ sub load_1 {
 
         for my $r (@repos) {
             for my $u (@users) {
-                push @rules, @{ $repos{$r}{$u} } if exists $repos{$r}{$u};
+                push @rules, @{ $repos{$r}{$u} } if exists $repos{$r} and exists $repos{$r}{$u};
             }
         }