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.
- delete a duplicate explanation of how git rebase skips commits with
  the same textual changes (it's explained in more detail a few lines
  further down)
- 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 <[email protected]>

Author: jvns

Documentation/git-rebase.adoc

@@ -65,31 +65,31 @@ linkgit:git-config[1] for details) and the `--fork-point` option is
 assumed.  If you are currently not on any branch or if the current
 branch does not have a configured upstream, the rebase will abort.
 
-All changes made by commits in the current branch but that are not
-in `<upstream>` are saved to a temporary area.  This is the same set
-of commits that would be shown by `git log <upstream>..HEAD`; or by
-`git log 'fork_point'..HEAD`, if `--fork-point` is active (see the
-description on `--fork-point` below); or by `git log HEAD`, if the
-`--root` option is specified.
-
-The current branch is reset to `<upstream>` or `<newbase>` if the
-`--onto` option was supplied.  This has the exact same effect as
-`git reset --hard <upstream>` (or `<newbase>`). `ORIG_HEAD` is set
-to point at the tip of the branch before the reset.
+Here is a more detailed description of what `git rebase <upstream>` does:
+
+First, it makes a list of all commits in the current branch that are not in
+`<upstream>`. This is the same set of commits that would be shown by `git log
+<upstream>..HEAD`. You can use `--fork-point` or `--root` to change how this
+list of commits is constructed.
+
+Then it checks out `<upstream>` (or `<newbase>` if the `--onto` option was
+supplied) with the equivalent of `git switch --detach <upstream>`.
+
+Then it replays the commits, one by one, in order. This is similar to running
+`git cherry-pick <commit>` for each commit. See REBASING MERGES for how merges
+are handled.
+
+Finally, it updates your branch to point to the final commit with the equivalent
+of `git switch -C <branch>`.
 
 [NOTE]
+`ORIG_HEAD` is set to point at the tip of the branch before the rebase.
 `ORIG_HEAD` is not guaranteed to still point to the previous branch tip
 at the end of the rebase if other commands that write that pseudo-ref
 (e.g. `git reset`) are used during the rebase. The previous branch tip,
 however, is accessible using the reflog of the current branch
 (i.e. `@{1}`, see linkgit:gitrevisions[7]).
 
-The commits that were previously saved into the temporary area are
-then reapplied to the current branch, one by one, in order. Note that
-any commits in `HEAD` which introduce the same textual changes as a commit
-in `HEAD..<upstream>` are omitted (i.e., a patch already accepted upstream
-with a different commit message or timestamp will be skipped).
-
 If the upstream branch already contains a change you have made (e.g.,
 because you mailed a patch which was applied upstream), then that commit
 will be skipped and warnings will be issued (if the 'merge' backend is