Simplify path stripping code

[JC3]

As per todo:

harextract/harextract.html

Lines 175 to 192 in b9a4dc0

	// now make a few passes to strip common path prefixes
	// todo: code can be simplified by maintaining common components above then
	// doing a single second pass to slice + join.
	if (harents.length > 0) {
	for (var ncommon = 0, allsame = true; allsame && ncommon < harents[0].pathcomps.length; ++ ncommon) {
	allsame = true;
	let refpart = harents[0].pathcomps[ncommon];
	for (ent of harents) {
	if (ent.pathcomps[ncommon] != refpart) {
	allsame = false;
	break;
	}
	}
	}
	for (ent of harents) {
	ent.path = (ncommon < 0 ? '' : ent.pathcomps.slice(ncommon - 1).join('/'));
	}
	}