diffhelpers: backport
9e40bc4c1bde from C implementation
9e40bc4c1bde just says "harden testhunk." I don't think this would be
the case, but it makes some sense to avoid negative index.
#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