project--completing-read-strict: Move some common processing here

project--completing-read-strict: Move some common processing here

* lisp/progmodes/project.el (project--completing-read-strict):
Add new optional argument, COMMON-PARENT-DIRECTORY.  Move the
absolute->relative processing of MB-DEFAULT and the contents of
HIST here.
(project--read-file-cpd-relative): From here.  So that
'project--read-file-absolute' can also benefit from those
conversions.
(project--read-file-absolute): Pass the new argument.
(project-read-file-name-function): Update value tags.

(cherry picked from commit c0cb369ab188ea7ae0d3271d19c0cecce7be0329)