doc: git-rebase: update discussion of internals

- make it clearer that we're talking about a multistep process
- give a more technically accurate description how rebase works with the
  merge backend.
- condense the explanation of how git rebase skips commits with the same
  textual changes into a single bullet point and remove the explanatory
  diagram. Lots of things which are more complicated are already being
  explained without a diagram.
- remove the explanation of how exactly `--fork-point` and `--root`
  work since that information is in the OPTIONS section
- put all discussion of `ORIG_HEAD` inside the note

Signed-off-by: Julia Evans <julia@jvns.ca>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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