emacs: Add interface for comparing generations.
Suggested by Ludovic Courtès.
* doc/emacs.texi (Emacs List buffer): Document new key bindings.
* emacs/guix-base.el (guix-generation-packages-buffer-name-function,
guix-generation-packages-update-buffer, guix-output-name-width): New
variables.
(guix-generation-file, guix-manifest-file, guix-generation-packages,
guix-generation-packages-buffer-name-default,
guix-generation-packages-buffer-name-long,
guix-generation-packages-buffer-name, guix-generation-packages-buffer,
guix-generation-insert-package, guix-generation-insert-packages,
guix-profile-generation-manifest-file,
guix-profile-generation-packages-buffer): New procedures.
* emacs/guix-list.el: Add key bindings for comparing generations.
(guix-generation-list-generations-to-compare,
guix-generation-list-show-added-packages,
guix-generation-list-show-removed-packages, guix-generation-list-compare,
guix-generation-list-ediff-manifests, guix-generation-list-diff-manifests,
guix-generation-list-ediff-packages, guix-generation-list-diff-packages,
guix-generation-list-ediff, guix-generation-list-diff): New procedures.
* emacs/guix-messages.el (guix-messages): Add 'generation-diff' search type.
(guix-message-outputs-by-diff): New procedure.
* emacs/guix-utils.el (guix-diff-switches): New variable.
(guix-diff): New procedure.
* emacs/guix-main.scm (package/output-sexps): Handle 'generation-diff' search
type.
(manifest-entry->package-specification,
manifest-entries->package-specifications, generation-package-specifications,
generation-package-specifications+paths, generation-difference): New
procedures.
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