pull: '--url', '--commit', and '--branch' apply to the 'guix' channel.

Suggested by pkill9 <pkill9@runbox.com>.

* guix/scripts/pull.scm (channel-list): Apply REF and URL to the 'guix'
channel.
* doc/guix.texi (Invoking guix pull): Adjust accordingly.

doc/guix.texi

@@ -3646,8 +3646,9 @@ but it supports the following options:
 @item --url=@var{url}
 @itemx --commit=@var{commit}
 @itemx --branch=@var{branch}
-Download code from the specified @var{url}, at the given @var{commit} (a valid
+Download code for the @code{guix} channel from the specified @var{url}, at the
-Git commit ID represented as a hexadecimal string), or @var{branch}.
+given @var{commit} (a valid Git commit ID represented as a hexadecimal
+string), or @var{branch}.
 
 @cindex @file{channels.scm}, configuration file
 @cindex configuration file for channels

guix/channels.scm

@@ -52,6 +52,7 @@
             channel-location
 
             %default-channels
+            guix-channel?
 
             channel-instance?
             channel-instance-channel

guix/scripts/pull.scm

@@ -502,24 +502,22 @@ Use '~/.config/guix/channels.scm' instead."))
         (url (or (assoc-ref opts 'repository-url)
                  (environment-variable))))
     (if (or ref url)
-        (match channels
+        (match (find guix-channel? channels)
-          ((one)
+          ((? channel? guix)
-           ;; When there's only one channel, apply '--url', '--commit', and
+           ;; Apply '--url', '--commit', and '--branch' to the 'guix' channel.
-           ;; '--branch' to this specific channel.
+           (let ((url (or url (channel-url guix))))
-           (let ((url (or url (channel-url one))))
+             (cons (match ref
-             (list (match ref
                      (('commit . commit)
-                      (channel (inherit one)
+                      (channel (inherit guix)
                                (url url) (commit commit) (branch #f)))
                      (('branch . branch)
-                      (channel (inherit one)
+                      (channel (inherit guix)
                                (url url) (commit #f) (branch branch)))
                      (#f
-                      (channel (inherit one) (url url)))))))
+                      (channel (inherit guix) (url url))))
-          (_
+                   (remove guix-channel? channels))))
-           ;; Otherwise bail out.
+          (#f                           ;no 'guix' channel, failure will ensue
-           (leave
+           channels))
-            (G_ "'--url', '--commit', and '--branch' are not applicable~%"))))
         channels)))