pg_combinebackup: When reconstructing, avoid double slash in filename.

pg_combinebackup: When reconstructing, avoid double slash in filename.

This function is always called with a relative_path that ends in a
slash, so there's no need to insert a second one. So, don't. Instead,
add an assertion to verify that nothing gets broken in the future, and
adjust the comments.

While this is not a critical bug, the duplicate slash is visible in
error messages, which could create confusion, so back-patch to v17.
This is also better in that it keeps the code consistent across
branches.

Patch by me, reviewed by Bertrand Drouvot and Amul Sul.

Discussion: http://postgr.es/m/CA+TgmoaD7dBYPqe7kMtO0dyto7rd0rUh7joh=JPUSaFszKY6Pg@mail.gmail.com