git-download: Print a message when falling back to a full fetch.

Otherwise the user might believe that git-fetch stalled, observing the lack of
output following a 'fatal' git error message (see:
https://debbugs.gnu.org/33100).

* guix/build/git.scm (git-fetch): Print message when falling back to a full
fetch.

guix/build/git.scm

@@ -45,6 +45,8 @@ recursively.  Return #t on success, #f otherwise."
     (if (zero? (system* git-command "fetch" "--depth" "1" "origin" commit))
         (invoke git-command "checkout" "FETCH_HEAD")
         (begin
+          (setvbuf (current-output-port) 'line)
+          (format #t "Failed to do a shallow fetch; retrying a full fetch...~%")
           (invoke git-command "fetch" "origin")
           (invoke git-command "checkout" commit)))
     (when recursive?