monad-repl: Add "graft?" command.

Grafts are not always required when just exploring in the REPL, and they do
take time, so this commit adds easy way to disable them.

* guix/monad-repl.scm (build-graft): New meta command.
* doc/guix.texi (Using Guix Interactively): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

doc/guix.texi

@@ -13265,6 +13265,11 @@ be a list of keywords with values accepted by procedure
 @end example
 @end deffn
 
+@deffn {REPL command} graft? @var{graft?}
+Set whether grafts should be performed as part of the build.  Equivalent
+to @code{--no-grafts} argument to @code{guix build}.
+@end deffn
+
 @deffn {REPL command} lower @var{object}
 Lower @var{object} into a derivation or store file name and return it.
 @end deffn

guix/monad-repl.scm

@@ -149,6 +149,11 @@ Must be a list of keywords with values accepted by procedure
   (repl-print repl %build-options)
   (set! %build-options (repl-eval repl opts)))
 
+(define-meta-command ((graft? guix) repl (graft?))
+  "build-graft GRAFT?
+Set whether grafts should be performed."
+  (%graft? (repl-eval repl graft?)))
+
 (define-meta-command ((enter-store-monad guix) repl)
   "enter-store-monad
 Enter a REPL for values in the store monad."