Suggested by Ludovic Courtès <ludo@gnu.org>.

* emacs/guix-build-log.el (guix-build-log-phase-end-regexp): New
  variable.
  (guix-build-log-phase-start, guix-build-log-phase-end,
  guix-build-log-phase-hide, guix-build-log-phase-show,
  guix-build-log-phase-hidden-p, guix-build-log-phase-toggle-function,
  guix-build-log-phase-toggle, guix-build-log-phase-toggle-all): New functions.
  (guix-build-log-mode-map): Add 'TAB'/'S-TAB' key bindings.
* doc/emacs.texi (Emacs Build Log): Document them.