merge: use collections.defaultdict() for mergeresult.commitinfo
We will be storing info from mergeresult.commitinfo to mergestate._stateextras
in upcoming patches, let's make them use same structure so that we don't have to
make much efforts in transferring info from one to other.
Differential Revision: https://phab.mercurial-scm.org/D8921
max_width = 79
wrap_comments = true
error_on_line_overflow = true