copies: delete now-unnecessary check for "a == b" before "a.descendant(b)"
Unnecessary since 879cbdde63df (revlog: do inclusive descendant
testing (API), 2018-06-21).
Differential Revision: https://phab.mercurial-scm.org/D3935
#require docutils gettext $ $TESTDIR/check-gendoc zh_CN checking for parse errors