Add special %GL_CREATOR variable for git-config

Add a special variable `%GL_CREATOR` to the the git-config trigger that
is replaced by the name of the repository creator (if any).

This can be useful to set up the default owner configuration for wild
repositories.

Example:

    repo assignments/CREATOR/a[0-9][0-9]
        C   = @students
        RW+ = CREATOR
        config gitweb.owner = %GL_CREATOR

----

committer added an if condition to the s/// line.

src/triggers/post-compile/update-git-configs

@@ -43,12 +43,14 @@ for my $pr (@$lpr) {
 
 sub fixup_config {
     my $pr = shift;
+    my $creator = creator($pr);
 
     my $gc = git_config( $pr, '.', 1 );
     while ( my ( $key, $value ) = each( %{$gc} ) ) {
         next if $key =~ /^gitolite-options\./;
         if ( $value ne "" ) {
             $value =~ s/%GL_REPO/$pr/g;
+            $value =~ s/%GL_CREATOR/$creator/g if $creator;
             system( "git", "config", "--file", "$RB/$pr.git/config", $key, $value );
         } else {
             system( "git", "config", "--file", "$RB/$pr.git/config", "--unset-all", $key );