[PATCH 2/3] t3404: rebase: interactive: demonstrate short SHA-1 collision

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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" is now
badbeef5ba78983324dff5265c80c4490d5a809a, 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@xxxxxxxxxxxxxx>
---
 t/t3404-rebase-interactive.sh | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/t/t3404-rebase-interactive.sh b/t/t3404-rebase-interactive.sh
index af141be..e5ebec6 100755
--- a/t/t3404-rebase-interactive.sh
+++ b/t/t3404-rebase-interactive.sh
@@ -977,4 +977,21 @@ 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 115158b5" collide collide3 collide3
+'
+
+test_expect_failure 'short SHA-1 collide' '
+	test_when_finished "reset_rebase && git checkout master" &&
+	git checkout collide &&
+	FAKE_COMMIT_MESSAGE="collide2 815200e" \
+	FAKE_LINES="reword 1 2" git rebase -i HEAD~2
+'
+
 test_done
-- 
1.8.4.rc2.460.ga591f4a

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