histedit: make histedit aware of obsolescense not stored in state (issue4800) Before this change, when histedit exited to interactive session (during edit command for example), user could introduce obsolescence markers that would not be known to histedit. For example, user could've amended one of the commits. The fact of this amendment would not be stored in histedit's state file and later, when histedit would try to process all the replacements, one of the final successors (in histedit's opinion) would turn out to be hidden. This behavior is described in issue4800. This commit fixes it.