t3404: rebase -i: demonstrate short SHA-1 collision

The 'todo' sheet for interactive rebase shows abbreviated SHA-1's and
then performs its operations upon those shortened values. This can lead
to an abort if the SHA-1 of a reworded or edited commit is no longer
unique within the abbreviated SHA-1 space and a subsequent SHA-1 in the
todo list has the same abbreviated value.

For example:

  edit f00dfad first
  pick badbeef second

If, after editing, the new SHA-1 of "first" also has prefix badbeef,
then the subsequent 'pick badbeef second' will fail since badbeef is no
longer a unique SHA-1 abbreviation:

  error: short SHA1 badbeef is ambiguous.
  fatal: Needed a single revision
  Invalid commit name: badbeef

Demonstrate this problem with a couple of specially crafted commits
which initially have distinct abbreviated SHA-1's, but for which the
abbreviated SHA-1's collide after a simple rewording of the first
commit's message.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3404-rebase-interactive.sh

@@ -1037,4 +1037,28 @@ test_expect_success 'rebase -i with --strategy and -X' '
 	test $(cat file1) = Z
 '
 
+test_expect_success 'short SHA-1 setup' '
+	test_when_finished "git checkout master" &&
+	git checkout --orphan collide &&
+	git rm -rf . &&
+	(
+	unset test_tick &&
+	test_commit collide1 collide &&
+	test_commit --notick collide2 collide &&
+	test_commit --notick collide3 collide
+	)
+'
+
+test_expect_failure 'short SHA-1 collide' '
+	test_when_finished "reset_rebase && git checkout master" &&
+	git checkout collide &&
+	(
+	unset test_tick &&
+	test_tick &&
+	set_fake_editor &&
+	FAKE_COMMIT_MESSAGE="collide2 ac4f2ee" \
+	FAKE_LINES="reword 1 2" git rebase -i HEAD~2
+	)
+'
+
 test_done