histedit: remove "should strip" debug message
The debug message was not used anywhere. Removed it to make
scmutil.cleanupnodes migration easier to reason about.
histedit: move topmost bookmark movement to a separate function
histedit treats topmost bookmark movement specially. The rest of the
bookmark movement could be handled by scmutil.cleanupnodes. So let's move
the special logic out to make the patch easier to review.