Make `cycle-spacing' behave more like `just-one-space' if colled once.

Make `cycle-spacing' behave more like `just-one-space' if colled once.

* simple.el (cycle-spacing): Never delete spaces on first run by
default, but do so in a new 'fast mode and if there are already
N spaces (the previous behaviour).

Compare N with its value in previous invocation so that changing
prefix argument restarts `cycle-spacing' sequence.

The idea is that with this change, binding M-SPC to
`cycle-spacing' should not introduce any changes in behaviour of
the binding so long as users do not type M-SPC twice in a raw with
the same prefix argument or lack thereof.