largefiles: get cwd and relative paths from repo instead of matcher

Authored by martinvonz.

Description

largefiles: get cwd and relative paths from repo instead of matcher

As far as I can tell, the matcher returned from matchandpats() got its
cwd from the context object, which in turn got it from the repo, so
this should be equivalent.

Differential Revision: https://phab.mercurial-scm.org/D5922