cp: copy dir if source path ends with dot

[cakebaker]

This PR makes the following work:

$ mkdir x
$ touch x/foo
$ cp -r x/. y # directory y is created with file foo

Fixes #7871

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/timeout/timeout (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/misc/stdbuf (passes in this run but fails in the 'main' branch)