histedit: mark temporary commits as obsolete when allowed to
Before this patch, we were stripping temporary commits at the end of a histedit
whether it was successful or not. If we can create obs markers, we should
create them instead of stripping because it is faster and safer.
histedit: minor refactoring of createmarkers check
We use a variable to store whether or not we can create obsolescence markers.
It makes the patch series more readable as we are going to reuse this
values in other places in the function.