Consider changing criteria for whether a branch is merged to parent if commits are equal

[PawelLipski]

Currently, if parent and child point to the same commit, we discriminate b/w green and gray edge based on filtered reflog (reflog without resets/branch creation) of the child: if it's empty, then green, else gray.

This is far from perfect, see https://github.com/VirtusLab/git-machete-intellij-plugin/pull/622#issue-505348624

An alternative could be to just check the timestamp of latest entry in parent's/child's reflog: if the latter is newer, then green, else green (?) This way, we'll check whether child has been "modified" more recently than parent; if it was, then likely the child shouldn't really be considered merged.