undo-files: cleanup backup when cleaning undos
Previously, the backups were left behind, by operation cleaning the undo's like
strip, narrow and stream clone.
The remaining elevant in the room is the transaction itself, who does not
properly cleanup undo backup before copying the new ones.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..