While commiting a merge, the commit code does not know whether a file was merged
during hg merge or not. This leads the commit code to look for filelog
ancestor to choose parents of new filelog created on merge commit.
This leads to wrong results in some cases as demonstrated by previous patch.
From this patch, we start storing information about merged files in mergestate
in stateextras and then use that on commit to detect whether we need to set two
parents or not.