guix build: '--with-branch' & co. fetch submodules.

* guix/scripts/build.scm (transform-package-source-branch)[replace]: Add
'recursive?' field to the new package.

doc/guix.texi

@@ -7289,7 +7289,8 @@ care!
 Build @var{package} from the latest commit of @var{branch}.  The @code{source}
 field of @var{package} must be an origin with the @code{git-fetch} method
 (@pxref{origin Reference}) or a @code{git-checkout} object; the repository URL
-is taken from that @code{source}.
+is taken from that @code{source}.  Git sub-modules of the repository are
+fetched, recursively.
 
 For instance, the following command builds @code{guile-sqlite3} from the
 latest commit of its @code{master} branch, and then builds @code{guix} (which

guix/scripts/build.scm

@@ -308,7 +308,8 @@ strings like \"guile-next=stable-3.0\" meaning that packages are built using
     (package
       (inherit old)
       (version (string-append "git." branch))
-      (source (git-checkout (url url) (branch branch)))))
+      (source (git-checkout (url url) (branch branch)
+                            (recursive? #t)))))
 
   (let* ((replacements (evaluate-git-replacement-specs replacement-specs
                                                        replace))