[PATCH v2 3/3] rebase -i: also avoid SHA-1 collisions with missingCommitsCheck

["Johannes Schindelin via GitGitGadget" <gitgitgadget@gmail.com>]

From: Johannes Schindelin <johannes.schindelin@gmx.de>

When `rebase.missingCommitsCheck` is in effect, we use the backup of the
todo list that was copied just before the user was allowed to edit it.

That backup is, of course, just as susceptible to the hash collision as
the todo list itself: a reworded commit could make a previously
unambiguous short commit ID ambiguous all of a sudden.

So let's not just copy the todo list, but let's instead write out the
backup with expanded commit IDs.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 rebase-interactive.c          | 8 +++++---
 t/t3404-rebase-interactive.sh | 2 ++
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/rebase-interactive.c b/rebase-interactive.c
index aa18ae82b7..1259adc8ea 100644
--- a/rebase-interactive.c
+++ b/rebase-interactive.c
@@ -104,9 +104,11 @@ int edit_todo_list(struct repository *r, struct todo_list *todo_list,
 				    -1, flags | TODO_LIST_SHORTEN_IDS | TODO_LIST_APPEND_TODO_HELP))
 		return error_errno(_("could not write '%s'"), todo_file);
 
-	if (initial && copy_file(rebase_path_todo_backup(), todo_file, 0666))
-		return error(_("could not copy '%s' to '%s'."), todo_file,
-			     rebase_path_todo_backup());
+	if (initial &&
+	    todo_list_write_to_file(r, todo_list, rebase_path_todo_backup(),
+				    shortrevisions, shortonto, -1,
+				    (flags | TODO_LIST_APPEND_TODO_HELP) & ~TODO_LIST_SHORTEN_IDS) < 0)
+		return error(_("could not write '%s'."), rebase_path_todo_backup());
 
 	if (launch_sequence_editor(todo_file, &new_todo->buf, NULL))
 		return -2;
diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index 1cc9f36bc7..b90ea0fe44 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -1277,6 +1277,8 @@ test_expect_success SHA1 'short SHA-1 collide' '
 			.git/rebase-merge/git-rebase-todo.tmp &&
 		grep "^pick [0-9a-f]\{40\}" \
 			.git/rebase-merge/git-rebase-todo &&
+		grep "^pick [0-9a-f]\{40\}" \
+			.git/rebase-merge/git-rebase-todo.backup &&
 		git rebase --continue
 	) &&
 	collide2="$(git rev-parse HEAD~1 | cut -c 1-4)" &&
-- 
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