In rgrep, check matching files before excluding files

There are a lot of excluding globs, and checking them all is expensive.
The files glob (i.e. the glob for files we actually want) is usually
just one or two entries, so it's quite fast to check.

If find checks the files glob first and then the excluding glob, it has
to do much less checking (since the files glob will substantially narrow
down the set of files on its own), and find performance is much better.

In my benchmarking, this takes (rgrep "foo" "*.el" "~/src/emacs/trunk/")
from ~410ms to ~130ms.

Further optimizations are possible now that the ignores and matched
files are in the same <F> argument which can be rearranged more easily
without compatibility issues; I'll do those optimizations in later
commits.

* lisp/progmodes/grep.el (rgrep-find-ignores-in-<f>): Add.
* lisp/progmodes/grep.el (rgrep-default-command): Check
rgrep-find-ignores-in-<f> and move the excluded files glob to part of
the "files" argument.  (Bug#71179)

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