Instead of having the service manage the list, it's useful to be able to point
this at an existing file, for example, when using cgit together with gitolite.
* gnu/services/cgit.scm (project-list?): New procedure.
(serialize-project-list): Handle file-object values.
(<cgit-configuration>): Change the predicate for project-list to allow lists
and file-objects.
* gnu/services/cgit.scm (serialize-cgit-configuration): Add the extra options,
one per line, before the scan-path, as this makes it possible to use the
extra-options to affect the global behaviour for repositories.
(serialize-extra-options): New procedure.
* doc/guix.texi (Version Control Services): Update accordingly.
* gnu/services/cgit.scm (cgit-configuration)[project-list]: New field.
(serialize-project-list): New procedure that uses PLAIN-FILE to generate a
file from the string list given by the user as input.
(serialize-cgit-configuration): Make sure to serialize 'project-list' before
'repostory-directory'.
* gnu/services/cgit.scm (serialize-cgit-configuration): New procedure that
serializes fields with a precise order.
(cgit-activation): Replace the generic SERIALIZE-CONFIGURATION with
SERIALIZE-CGIT-CONFIGURATION.
