bug#29659: 25.2; better rmail-summary-by-topic

[Francesco Potortì]

rmail-summary-by-topic functionality can be easily improved by this
piece of advice:

(defun rmail-simplified-subject--strip-all (subject)
  "Strip all prefixes like Re:, Fwd: and similar ones"
  (let ((search-spaces-regexp "[ \t\n]"))
    (string-match " *\\(\\(\\w\\{1,3\\}:\\|\\[.+]\\) +\\)*" subject)
    (replace-match "" t t subject))
  )
(advice-add 'rmail-simplified-subject :filter-return 
#'rmail-simplified-subject--strip-all)


Maybe this can be simply incorporated into rmail-simplified-subject: it
is able to remove prefixes like

Re: [Topic] Fwd: Re: Fwd:

and in practice I find it very useful