Importer: If the serializer wrote absoolute paths, be sure to trim them

3d9985d8 trimmed the stored path from the list of files, but only the
first level, and only functioned if the directory was relative, not
absolute.  Trim and replace the entire path, which makes archives
created with `rt-serialize --directory /tmp/rt` load correctly.

Closes [issues #28434].
Merge branch '4.2/importer-abs-paths' into 4.2-trunk