tests: avoid the word "dirty" to mean "not a descendant of merge base"
The term "dirty" is no longer used in the code since 57203e0210f8
(copies: calculate mergecopies() based on pathcopies(), 2019-04-11).
Differential Revision: https://phab.mercurial-scm.org/D6373
#require docutils gettext
$ $TESTDIR/check-gendoc pt_BR
checking for parse errors