subrepo: use root-repo-relative path from `hg files` with ui.relative-paths=no
The fix is to pass in a "subuipathfn" as we do everywhere else.
Differential Revision: https://phab.mercurial-scm.org/D5978
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint