rebase -i: re-fix short SHA-1 collision

In 66ae9a57b88 (t3404: rebase -i: demonstrate short SHA-1 collision,
2013-08-23), we added a test case that demonstrated how it is possible
that a previously unambiguous short commit ID could become ambiguous
*during* a rebase.

In 75c69766554 (rebase -i: fix short SHA-1 collision, 2013-08-23), we
fixed that problem simply by writing out the todo list with expanded
commit IDs (except *right* before letting the user edit the todo list,
in which case we shorten them, but we expand them right after the file
was edited).

However, the bug resurfaced as a side effect of 393adf7a6f6 (sequencer:
directly call pick_commits() from complete_action(), 2019-11-24): as of
this commit, the sequencer no longer re-reads the todo list after
writing it out with expanded commit IDs.

The only redeeming factor is that the todo list is already parsed at
that stage, including all the commits corresponding to the commands,
therefore the sequencer can continue even if the internal todo list has
short commit IDs.

That does not prevent problems, though: the sequencer writes out the
`done` and `git-rebase-todo` files incrementally (i.e. overwriting the
todo list with a version that has _short_ commit IDs), and if a merge
conflict happens, or if an `edit` or a `break` command is encountered, a
subsequent `git rebase --continue` _will_ re-read the todo list, opening
an opportunity for the "short SHA-1 collision" bug again.

To avoid that, let's make sure that we do expand the commit IDs in the
todo list as soon as we have parsed it after letting the user edit it.

Additionally, we improve the 'short SHA-1 collide' test case in t3404 to
test specifically for the case where the rebase is resumed. We also
hard-code the expected colliding short SHA-1s, to document the
expectation (and to make it easier on future readers).

Note that we specifically test that the short commit ID is used in the
`git-rebase-todo.tmp` file: this file is created by the fake editor in
the test script and reflects the state that would have been presented to
the user to edit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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