Fixes <https://issues.guix.gnu.org/43893>.
This changes the 'update-guix-package' tool so that it:
1. Always uses a clean checkout to compute the hash of the updated 'guix'
package.
2. Ensures the commit used in the updated 'guix' package definition has already
been pushed upstream.
* build-aux/update-guix-package.scm (%savannah-guix-git-repo-push-url): New
variable.
(with-input-pipe-to-string, with-temporary-git-worktree): New syntaxes.
(find-origin-remote, git-add-worktree): New procedures.
(commit-already-pushed?): New predicate.
(main): Check the commit used has already been pushed upstream and compute the
hash from a clean checkout.
* doc/contributing.texi (Updating the Guix Package): Document it.
* .dir-locals.el (scheme-mode): Fix indentation of with-temporary-git-worktree.
Previously we'd pass a relative file name to 'add-indirect-root', which
the daemon would interpret as relative to "/". Consequently, checkouts
were not protected from GC.
* build-aux/update-guix-package.scm (main): Pass an absolute file name
to 'add-indirect-root'.
* build-aux/update-guix-package.scm: New file.
* Makefile.am (EXTRA_DIST): Add it.
(update-guix-package): New target.
(.PHONY): Add it.
* gnu/packages/package-management.scm (guix): Mention it.
