rebase -i -p: include non-first-parent commits in todo list

rebase -i -p: include non-first-parent commits in todo list

Consider this graph:

        D---E    (topic, HEAD)
       /   /
  A---B---C      (master)
   \
    F            (topic2)

and the following three commands:
  1. git rebase -i -p A
  2. git rebase -i -p --onto F A
  3. git rebase -i -p B

Currently, (1) and (2) will pick B, D, C, and E onto A and F,
respectively.  However, (3) will only pick D and E onto B, but not C,
which is inconsistent with (1) and (2).  As a result, we cannot modify C
during the interactive-rebase.

The current behavior also creates a bug if we do:
  4. git rebase -i -p C

In (4), E is never picked.  And since interactive-rebase resets "HEAD"
to "onto" before picking any commits, D and E are lost after the
interactive-rebase.

This patch fixes the inconsistency and bug by ensuring that all children
of upstream are always picked.  This essentially reverts the commit:
  d80d6bc146232d81f1bb4bc58e5d89263fd228d4

When compiling the todo list, commits reachable from "upstream" should
never be skipped under any conditions.  Otherwise, we lose the ability
to modify them like (3), and create a bug like (4).

Two of the tests contain a scenario like (3).  Since the new behavior
added more commits for picking, these tests need to be updated to
account for the additional pick lines.  A new test has also been added
for (4).

Signed-off-by: Andrew Wong <andrew.kw.w@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>