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 <[email protected]>

Author: jvns

Documentation/git-rebase.adoc

@@ -68,51 +68,28 @@ 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 simplified description of what `git rebase <upstream>` does:
+
+1. Make 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.
+2. Check whether any of those commits are duplicates of commits already
+   in `<upstream>` and remove them from the list.
+3. Check out `<upstream>` with the equivalent of `git checkout --detach <upstream>`.
+4. Replay 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.
+5. Update your branch to point to the final commit with the equivalent
+   of `git checkout -C <branch>`.
 
 [NOTE]
-`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
-used).  For example, running `git rebase master` on the following
-history (in which `A'` and `A` introduce the same set of changes, but
-have different committer information):
-
-------------
-          A---B---C topic
-         /
-    D---E---A'---F master
-------------
-
-will result in:
-
-------------
-                   B'---C' topic
-                  /
-    D---E---A'---F master
-------------
+When starting the rebase, `ORIG_HEAD` is set to point to the commit at the tip
+of the to-be-rebased branch. However, `ORIG_HEAD` is not guaranteed to still
+point to that commit at the end of the rebase if other commands that change
+`ORIG_HEAD` (like `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].
 
 MODE OPTIONS
 ------------