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).
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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
