rebase -i: do not invent onelines when expanding/collapsing SHA-1s

To avoid problems with short SHA-1s that become non-unique during the
rebase, we rewrite the todo script with short/long SHA-1s before and
after letting the user edit the script. Since SHA-1s are not intuitive
for humans, rebase -i also provides the onelines (commit message
subjects) in the script, purely for the user's convenience.

It is very possible to generate a todo script via different means than
rebase -i and then to let rebase -i run with it; In this case, these
onelines are not required.

And this is where the expand/collapse machinery has a bug: it *expects*
that oneline, and failing to find one reuses the previous SHA-1 as
"oneline".

It was most likely an oversight, and made implementation in the (quite
limiting) shell script language less convoluted. However, we are about
to reimplement performance-critical parts in C (and due to spawning a
git.exe process for every single line of the todo script, the
expansion/collapsing of the SHA-1s *is* performance-hampering on
Windows), therefore let's fix this bug to make cross-validation with the
C version of that functionality possible.

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

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