vendor/libedit: upgrade from 2019-03-24 to 2019-12-31

vendor/libedit: upgrade from 2019-03-24 to 2019-12-31

Most significant changes:

- avoid screwed terminal when going back in history [1]
- fix undefined terminal behavior in terminal_move_to_line() [2]
- avoid uninitialized memory access in hist_get() [3]
- fix segfault from re_fastputc() in certain situations [4]

[1] http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=52359
[2] http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=54329
[3] http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=54399
[4] http://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=54654