mergeresult: implement a len() function and use it
In next patch we will start yielding from `getactions()` instead of building and
returning a list. Hence we can no longer rely on that for getting us a length.
Differential Revision: https://phab.mercurial-scm.org/D8880
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors