* lisp/ob-core.el (org-babel-expand-noweb-references): Add support for
`noweb-prefix' header argument, to not repeat the prefix characters
when expanding a noweb reference.
(org-babel-common-header-args-w-values):
(org-babel-safe-header-args): Add `noweb-prefix' value.
* doc/org-manual.org: Document `noweb-prefix' babel header argument.
* etc/ORG-NEWS: Document `:noweb-prefix'.