services: cgit: Make project-list permit a file-object.

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.

doc/guix.texi

@@ -19241,7 +19241,7 @@ Defaults to @samp{#f}.
 
 @end deftypevr
 
-@deftypevr {@code{cgit-configuration} parameter} list project-list
+@deftypevr {@code{cgit-configuration} parameter} project-list project-list
 A list of subdirectories inside of @code{repository-directory}, relative
 to it, that should loaded as Git repositories.  An empty list means that
 all subdirectories will be loaded.

gnu/services/cgit.scm

@@ -116,6 +116,10 @@
 (define (serialize-file-object field-name val)
   (serialize-string field-name val))
 
+(define (project-list? val)
+  (or (list? val)
+      (file-object? val)))
+
 
 ;;;
 ;;; Serialize <nginx-server-configuration>
@@ -168,7 +172,9 @@
   (if (null? val) ""
       (serialize-field
        'project-list
-       (plain-file "project-list" (string-join val "\n")))))
+       (if (file-object? val)
+           val
+           (plain-file "project-list" (string-join val "\n"))))))
 
 (define (serialize-extra-options extra-options)
   (string-join extra-options "\n" 'suffix))
@@ -547,7 +553,7 @@ disabled.")
    "Flag which, when set to @samp{#t}, will make cgit omit the standard
 header on all pages.")
   (project-list
-   (list '())
+   (project-list '())
    "A list of subdirectories inside of @code{repository-directory}, relative
 to it, that should loaded as Git repositories.  An empty list means that all
 subdirectories will be loaded.")