tests: replace `tac` reimplementation by `sort -r`
`sort -r` is better code "formatter" than `tac` since it's
stable. It's also portable so we don't need to reimplement it in
Python.
Differential Revision: https://phab.mercurial-scm.org/D5239
#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