Possible issue with rebase's --rebase-merges option

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What did you do before the bug happened? (Steps to reproduce your issue) This is a difficult one to give proper steps to reproduce. The issue is with rebase's --rebase-merges flag. We recently switched from using rebase with the --preserve-merges option to --rebase-merges. Most of the time the output is the same, but sometimes it is very different. I'm unable to determine whether this is by design or a bug.

What did you expect to happen? (Expected behavior) Resulting graph after running rebase --rebase-merges is the same as running rebase --preserve-merges.

What happened instead? (Actual behavior) Using --rebase merges tries to pick substantially more commits and results in merge commits with no parent commit when viewing log in reverse chronological order.

What's different between what you expected and what actually happened? When the issue does occur (it doesn't for all rebases) it results in two completely different logs and picks commits that are apparently not part of the branch being rebased. eg, for a branch with 128 commits including merges, --preserve-merges picks 128 commits and the resulting topology matches the original branch's topology. --rebase-merges picked 183(?) commits in v2.24 and 202 commits in v2.27, and in both cases resulted in a very strange topology.

Anything else you want to add: Feel free to contact me at joel@tusksoft.com for additional details. I would love to understand if this is by design or a legitimate bug.

Please review the rest of the bug report below. You can delete any lines you don't wish to share.

[System Info] git version: git version 2.27.0.windows.1 cpu: x86_64 built from commit: 907ab1011dce9112700498e034b974ba60f8b407 sizeof-long: 4 sizeof-size_t: 8 uname: Windows 10.0 18363 compiler info: gnuc: 10.1 libc info: no libc information available

[Enabled Hooks]