(major change in big-config mode) split the compiled config file

Fedora's config has over 11,000 repositories and the compiled config
file is over 20 MB in size.  Although negligible on a server class
machine, on my laptop just parsing this file takes a good 2.5 seconds.

Even if you use GL_ALL_READ_ALL (see a couple of commits before this
one) to remove the overhead for 'read's, that's still a pretty big
overhead for writes.  And GL_ALL_READ_ALL is not really a solution for
most people anyway.

With this commit, using GL_BIG_CONFIG adds another optimisation; see
doc/big-config.mkd for details (look for the word "split config" to find
the section that talks about it).

----

Implementation notes:

  - the check for GL_NO_CREATE_REPOS has moved *into* the loop (which it
    completely bypassed earlier) so that write_1_compiled_conf can be
    called on each item

t/out/t01-repo-groups.2

@@ -23,7 +23,21 @@ $data_version = '1.7';
       '@g1' => 1,
       '@g2' => 1
     }
-  },
+  }
+);
+%groups = (
+  '@g1' => {
+    'aa' => 'master',
+    'bb' => 'master'
+  }
+);
+%split_conf = (
+  'gitolite-admin' => 1,
+  'testing' => 1
+);
+repositories/gitolite-admin.git/gl-conf
+repositories/testing.git/gl-conf
+%one_repo = (
   'gitolite-admin' => {
     'R' => {
       'tester' => 1
@@ -38,7 +52,9 @@ $data_version = '1.7';
         'RW+'
       ]
     ]
-  },
+  }
+);
+%one_repo = (
   'testing' => {
     '@all' => [
       [
@@ -55,9 +71,3 @@ $data_version = '1.7';
     }
   }
 );
-%groups = (
-  '@g1' => {
-    'aa' => 'master',
-    'bb' => 'master'
-  }
-);