merge: move initial handling of mergeactions near to later one
We build `mergeactions` in the beginning and use it in end. Let's build it just
before where it will be used. Helps making code much easier to understand.
Differential Revision: https://phab.mercurial-scm.org/D8983
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors