Improve and extend admin/git-bisect-start

* admin/git-bisect-start: Use 'git bisect skip' instead of 'git
bisect good' for merges of external trees, which is equivalent but
faster in some cases.  Improve the explanation on external tree
merges.  Add list of commits in the last six years on which
building Emacs fails, and call 'git bisect skip' on them.  Also
document how the script can be called automatically when 'git
bisect start' is called in the Emacs repository.

* admin/notes/repo (Bisecting): Adapt the explanation of the
script accordingly.

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