Allow customizing the display of project file names when reading
To hopefully resolve a long-running discussion (https://lists.gnu.org/archive/html/emacs-devel/2019-05/msg00162.html). * lisp/progmodes/project.el (project-read-file-name-function): New variable. (project--read-file-absolute, project--read-file-cpd-relative): New functions, possible values for the above. (project-find-file-in): Use the introduced variable. (project--completing-read-strict): Retain just the logic that fits the name.