C-hkSPC tells us:
SPC runs the command minibuffer-complete-word (found in
minibuffer-local-completion-map), which is an interactive compiled
Lisp function in ‘minibuffer.el’.
Complete the minibuffer contents at most a single word.
After one word is completed as much as possible, a space or hyphen
is added, provided that matches some possible completion.
Return nil if there is no valid completion, else t.
Which firmly suggests that changing this particular binding is
going to affect more than just
rgrep prompts. Still, you can do this as follows if you wish to:
(define-key minibuffer-local-completion-map (kbd "SPC") nil)
You've already mentioned one workaround with C-qSPC
but there's another one which is almost as easy to type as SPC
cycle-spacing which, in the absence of any
space at point, inserts a single space. YMMV, but this binding is sufficiently convenient for me to not care when
SPC has a non-space binding.