Otherwise the global counterpart is never taken into account.
* doc/guix.texi (Version Control Services): Update accordingly.
* gnu/services/cgit.scm (repo-boolean?, serialize-repo-boolean): Use the
DEFINE-MAYBE macro to allow for the 'disabled value.
(repository-cgit-configuration)[enable-commit-graph?, enable-log-filecount?,
enable-log-linecount?, enable-remote-branches?, enable-subject-links?,
enable-html-serving?]: Change default value to 'disabled.
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