got-notify-http: fix unicode handling

got-notify-http: fix unicode handling

JSON strings are made of UNICODE codepoints, of which only \, " and
control characters have to be escaped, and the whole document MUST
be encoded in UTF-8.  The current code generates invalid strings
for non-ASCII characters, so it has to be made UTF-8 aware.

tedu' isu8cont() can't be used since it allows surrogate pairs and
overlong sequences which will cause decoding errors on the receiving
side.  Similarly, mbtowc() depends on the current locale and could
cause issues in -portable.

Instead, bundle Björn Höhrmann's "Flexible and Economical UTF-8
Decoder" and use it to parse the text.  Decoding errors results in
the replacement character U+FFFD being emitted and the bytes
considered so far to be discarded; the decoder is then restarted
with the next byte.

Git commit messages don't carry the notion of the encoding, but
it's reasonable to expect UTF-8 (which is a superset of ASCII).
For other more esotic encodings, the commit id can be used to
manually extract the data.

ok stsp@