largefiles: drop "rel" prefix from filename variables

Authored by martinvonz.

Description

largefiles: drop "rel" prefix from filename variables

The prefixes were meant to indicate that these paths are repo-relative
as opposed to absolute. However, that's what the majority of paths in
our code base are, so "rel" made me think they were instead
cwd-relative. Let's just drop the prefixes.

Differential Revision: https://phab.mercurial-scm.org/D5847

Details