Fix handling of Thai tis620-0 input encoding.

Fix handling of Thai tis620-0 input encoding.

The Thai tis620-0 input encoding is supported via the inputenc "plug in"
(data) file tis620.def from https://ctan.org/pkg/babel-thai.

We can handle it like the other contributed input encodings, e.g.,
Greek (ISO 8859-7) and the several Cyrillic encodings from
http://www.ctan.org/pkg/latex-cyrillic.

Under TeXLive 2018, the input encoding defaults to utf8, if there is no call to
inputenc. The added test file fails without the patch but compiles fine, if the
file "tis620.def" is present in the TEXPATH.