rebase -i: fix possibly wrong onto hash in todo

rebase -i: fix possibly wrong onto hash in todo

'todo_list_write_to_file' may overwrite the static buffer, originating
from 'find_unique_abbrev', that was used to store the short commit hash
'c' for "# Rebase a..b onto c" message in the todo editor. This is
because the buffer that is returned from 'find_unique_abbrev' is valid
until 4 more calls to `find_unique_abbrev` are made.

As 'todo_list_write_to_file' calls 'find_unique_abbrev' for each rebased
commit, the hash for 'c' is overwritten if there are 4 or more commits
in the rebase. This behavior has been broken since its introduction.

Fix by storing the short onto commit hash in a different buffer that
remains valid, before calling 'todo_list_write_to_file'.

Found-by: Jussi Keränen <redacted>
Signed-off-by: Antti Keränen <redacted>
Acked-by: Alban Gruin <redacted>
Signed-off-by: Junio C Hamano <redacted>