[PATCH] rebase: introduce --update-branches option

Sometimes people have to rebase multiple related branches. One way to do that quickly, when there are branches pointing to ancestors of a later branch (which happens a lot if you try hard to pad your PR count on GitHub--I mean if you try to make small, logically separate changes), is to rebase that later branch and then reset ancestor branches to the rewritten commits. You just have to work out which branches correspond to which of the new commits.

Here's an automated way to update those ancestor branches.

It's implemented as a function that processes a todo list, modeled after `todo_list_add_exec_commands`. Currently steps are added as `exec git branch -f <branchname>`, which comes with the caveat that they're not applied atomically when it finishes rebasing.

If you were to use this feature to rebase `my-dev` onto `master` in this situation:

```
 A - B          (master)
  |\
  |  E          (feat-e)
   \   \
     F  |       (feat-f)
       \|
         G      (interim)
           \
             H  (my-dev)
```

you'd get a todo list like this:

``` label onto

# Branch G reset onto pick 65945ab E exec git branch -f feat-e label G

reset onto pick 4f0b0ad F exec git branch -f feat-f merge -C e50066c G # G exec git branch -f interim pick 539e556 H ```

Warren He (1):
  rebase: introduce --update-branches option

 Documentation/git-rebase.txt      |  8 +++++
 builtin/rebase.c                  | 13 ++++++--
 sequencer.c                       | 68 ++++++++++++++++++++++++++++++++++++++-
 sequencer.h                       |  6 ++--
 t/t3431-rebase-update-branches.sh | 64 ++++++++++++++++++++++++++++++++++++
 5 files changed, 154 insertions(+), 5 deletions(-)
 create mode 100755 t/t3431-rebase-update-branches.sh