* doc/guix.texi (Version Control Services): Update accordingly.
* gnu/services/cgit.scm (serialize-field, serialize-string, serialize-boolean,
serialize-integer, serialize-repository-cgit-configuration-list,
serialize-nginx-server-configuration-list, serialize-repo-field,
serialize-repo-boolean, serialize-repo-integer, serialize-module-link-path,
serialize-repository-directory, serialize-mimetype-alist): Return strings or
string-valued gexps and stop printing.
(repository-cgit-configuration)[source-filter, about-filter, commit-filter,
logo, owner-filter], (cgit-configuration)[auth-filter, commit-filter, css,
email-filter, favicon, include, logo, owner-filter, mimetype-file, readme,
source-filter]: Replace STRING with FILE-OBJECT.
(file-object?, serialize-file-object, repo-file-object?,
serialize-repo-file-object): New procedures.
(cgit-activation): Use SERIALIZE-CONFIGURATION's return value with
MIXED-TEXT-FILE instead of using its output with PLAIN-FILE.