patch: move yielding "\n" to the end of loop
The original logic makes it harder to reason about - it yields the "\n"
character belonging to the last line in the next loop iteration.
The new code is in theory a little bit slower. But is more readable. It
makes the following changes easier to read.
Differential Revision: https://phab.mercurial-scm.org/D3210
#require docutils gettext
$ $TESTDIR/check-gendoc ru
checking for parse errors