maint: update-guix-package: Optionally add sources to store.

maint: update-guix-package: Optionally add sources to store.

Following discussions in <https://issues.guix.gnu.org/43893>, keeping a copy
of the updated package source is desirable when generating a release.

* build-aux/update-guix-package.scm (version-controlled?): Remove variable.
(call-with-temporary-git-worktree): Renamed from
'with-temporary-git-worktree'.  Update doc.  Do not change directory
implicitly.  Define as a procedure, not a syntax.
(keep-source-in-store): New procedure.
(main): Adjust to use with call-with-temporary-git-worktree.  Add the sources
to the store when GUIX_ALLOW_ME_TO_USE_PRIVATE_COMMIT is set.  Exit gracefully
when FIND-ORIGIN-REMOTE returns #f.
(%savannah-guix-git-repo-push-url-regexp): Adjust match for a potential colon
separator.
* Makefile.am (GUIX_ALLOW_ME_TO_USE_PRIVATE_COMMIT): Adjust.
* .dir-locals.el (scheme-mode): Remove entry for with-temporary-git-worktree.
* doc/contributing.texi (Updating the Guix Package): Update doc.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>