Allow refining the *Find* buffer of find-dired. (Bug#29513)

* find-dired.el (find-dired-refine-function): New user variable.
(find-dired-sentinel): Use it.  Simplify.
(find-dired-sort-by-filename): New function.
1 job for master in 3 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #2114


Name Stage Failure
test-all Test There has been a runner system failure, please try again
Running with gitlab-runner 11.6.0 (f100a208)
on gitlab-runner-docker cff9d373
Using Docker executor with image debian:stretch ...
Pulling docker image debian:stretch ...
Using docker image sha256:e1de74e67cc70422153600785c4ce4c29843a0e59e18b634c52ca133c564437f for debian:stretch ...
ERROR: Job failed (system failure): Error response from daemon: failed to start shim: exec: "docker-containerd-shim": executable file not found in $PATH: unknown (executor_docker.go:1018:0s)