Bug with branches/merges in submodules

Hello, I ran into a bug where commits are omitted from `git submodule summary` and friends when the submodule contains merge commits. Originally using 2.30.2, I can also reproduce this with a fresh build on the `next` branch (see attached bugreport). I would assume that this is not intentional, however I cannot find any relevant information on this. Feel free to contact me for extra details. Best regards, Mel

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue) Set up a submodule that has branches/merge commits: ```bash # create repository "child" mkdir child-repo cd child-repo git init touch initial.txt git add initial.txt git commit -m "initial child" cd .. # create repository "parent" mkdir parent cd parent git init # add submodule "child" to repository "parent" git submodule add ../child-repo/ child git commit -am "initial parent" cd child # make two commits in separate branches in "child" git switch -c "secondary" touch s1.txt git add s1.txt git commit -m "s1" git switch master touch m1.txt git add m1.txt git commit -m "m1" # merge branch "secondary" into "master" in "child" - this creates a merge commit git merge secondary --no-edit cd .. ``` Run `git diff --submodule=log` inside the "parent" repository.

What did you expect to happen? (Expected behavior) `git diff --submodule=log` should show all three commits added to the "child" submodule: ``` Submodule child XXXXXXX..YYYYYYY

```

What happened instead? (Actual behavior) `git diff --submodule=log` only shows commits from one ancestor of the merge commit: ``` Submodule child XXXXXXX..YYYYYYY

```

What's different between what you expected and what actually happened? All the commits added to the "secondary" branch are missing in `git diff --submodule=log`.

Anything else you want to add: The commit range shown by `git diff --submodule=log` is correct: `cd child; git log XXXXXXX..YYYYYYY` shows the correct list of commits.

This bug also affects `git submodule summary`, and `git show --submodule=log` after the changes have been committed.

[System Info] git version: git version 2.32.0.262.g8c582fcd64 cpu: x86_64 built from commit: 8c582fcd643d12802e0a6d7e307890aa5b6b26e1 sizeof-long: 8 sizeof-size_t: 8 shell-path: /bin/sh uname: Linux 5.11.0-22-generic #23-Ubuntu SMP Thu Jun 17 00:34:23 UTC 2021 x86_64 compiler info: gnuc: 10.3 libc info: glibc: 2.33 $SHELL (typically, interactive shell): /bin/bash

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