bug#380: previous-matching-history-element beef up

[jidanni]

Let's examine today the docstring for

   M-r (translated from <escape> r) runs the command 
previous-matching-history-element
     which is an interactive compiled Lisp function in `simple.el'.
   It is bound to M-r.
   (previous-matching-history-element REGEXP N)

   Find the previous history element that matches REGEXP.
   (Previous history elements refer to earlier actions.)
   With prefix argument N, search for Nth previous match.
   If N is negative, find the next or Nth next match.
   Normally, history elements are matched case-insensitively if
   `case-fold-search' is non-nil, but an uppercase letter in REGEXP
   makes the search case-sensitive.
   See also `minibuffer-history-case-insensitive-variables'.

OK, but what about when you want to search further back again: mention that you
just hit a second M-r RET...

And when one does hit that second M-r, it would be nice if the prompt
would show what the current default search string is. All it ever says is
"Previous element matching (regexp): " though indeed it remembers a
default all the time at least after first use.

Indeed it might even also say "found on history item 432" or "found at
18% of history" upon finding something, but maybe that's too verbose.

Also some of us would like C-r to "bust through" into previous lines,
so we don't have to use the less familiar M-r (ESC r for us old dogs
who never learned new ALT tricks). Maybe make a variable to allow
that.

All the above also applies to M-s.