git-rebase produces incorrect output

[Pavel Roskin <plroskin@gmail.com>]

Hi!

I've discovered an issue with "git rebase" producing a subtly
incorrect file. In fact, that files even compiled but failed in unit
tests! That's so scary that I'm going to stop using "git rebase" for
now. Fortunately, "git rebase --merge" is working correctly, so I'll
use it. Too bad there is no option to use "--merge" by default.

The issue was observed in git 2.23 and reproduced in today's next
branch (2.24.0.449.g4c06f74957) on up-to-date Fedora 31 x86_64.

I've created a repository that demonstrates the issue:
https://github.com/proski/git-rebase-demo

The branch names should be self-explanatory. "master" is the base,
"branch1" and "branch2" contain one change each. If "branch1" is
rebased on top of "branch2", the result is incorrect, saved in the
"rebase-bad" branch. If "git rebase -m" is used, the result is
correct, saved in the "merge-good" branch.

The files in "rebase-bad" and "merge-good" have exactly the same lines
but in a different order. Yet the changes on branch1 and branch2
affect non-overlapping parts of the file. There should be no doubt how
the merged code should look like.

I believe the change on branch2 shifts the lines, so that the first
change from branch1 is applies to a place below the intended location,
and then git goes back to an earlier line to apply the next hunk. I
can imagine that it would do the right thing in case of swapped blocks
of code. Yet I have a real life example where it does a very wrong
thing.

Indeed, "git diff origin/branch2 origin/rebase-bad" and "git diff
origin/branch2 origin/merge-good" both produce diffs of 9957 bytes
long, different only in the order of the hunks.

Another interesting data point - "git rebase --interactive" is working
correctly.

-- 
Regards,
Pavel Roskin