histedit: fix silly bug that was unpacking a bytestr before writing it
I have this foggy notion that popbuffer() might have returned a list
in the past, but it doesn't anymore, and this was breaking on Python
3. As a bonus, it's probably a ton faster on Python 2 now.
Differential Revision: https://phab.mercurial-scm.org/D2243
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors