Bug: merge-driver not executed with ort and merge-tree

Hi Git community,

we’re using a merge driver to automatically solve predictable merge conflicts in pom.xml files.

In our use-case Git is executed by Bitbucket. With the update of Bitbucket from 7.x to 8.x and Git from 2.23 to 2.43.2 the merge driver integration breaks.

Bitbucket internally calls “merge-tree” on a Git repo without working directory (dirty details below). While digging down the rabbit hole we found the following two bugs / inconsistencies:

* The "new" default merge strategy ort implements its own lookup logic for the .gitattributes file and ignores HEAD:.gitattributes.
  This behaviour is inconsistent to the default behaviour "-s recursive" prior to 2.34.
* merge-tree ignores HEAD:.gitattributes.

*Expected behavior:* merge-tree and “ort” respect HEAD:.gitattributes

Steps to reproduce (aligned with the calls from Bitbucket, so please don’t blame me for the calls 😊): You’ll find a demo repo and the instructions in https://github.com/afrimberger/mergedriver-test/blob/main/README.md

------------------------------------------------------------------------------------------------------------------------ ```shell $ git --version git version 2.43.2 ```

Clone this repo as bare repo to simulate Bitbucket's local repository storage

```shell $ cd /tmp $ git clone --bare https://github.com/afrimberger/mergedriver-test.git mergedriver-test-bare ```

Copy and activate the test merge driver ```shell $ cd /tmp/mergedriver-test-bare $ git show HEAD:custom-merge-driver.sh > /tmp/custom-merge-driver.sh $ chmod 777 /tmp/custom-merge-driver.sh $ /tmp/custom-merge-driver.sh

Merge Driver is active

```

Define the merge driver settings in one of the "global" Git configs
```shell
$ cat <<EOF >> ~/.gitconfig
[user]
  name = John Doe
  email = john@doe.com
[merge "keeplocalversion"]
  name = "Keep local pom version"
  driver = /tmp/custom-merge-driver.sh
EOF
```

Create temporary Git repo to prevent changes in bare repo

```shell $ mkdir /tmp/mergedriver-test $ cd /tmp/mergedriver-test $ git init Initialized empty Git repository in /tmp/mergedriver-test/.git/ ```

Reference objects from bare repo ```shell $ echo "/tmp/mergedriver-test-bare/objects" > ".git/objects/info/alternates" ```

The working directory is empty ```shell $ ls -al total 12 drwxr-xr-x 3 root root 4096 Feb 23 18:10 . drwxrwxrwt 1 root root 4096 Feb 23 18:10 .. drwxr-xr-x 8 root root 4096 Feb 23 18:10 .git ```

Prepare merge

```shell $ git reset 4814fce142b95b910d0cbe87c3c304b15f279a0a – Unstaged changes after reset: D .gitattributes D .gitconfig-example D custom-merge-driver.sh D pom.xml ```

working directory is empty, but `HEAD:.gitattributes` exists
```shell
$ git show HEAD:.gitattributes
* merge=keeplocalversion
```

Execute `git merge-tree`. The merge driver isn't executed even though `HEAD:.gitattributes` exists. ```shell $ git merge-tree --write-tree --allow-unrelated-histories --messages 561da86fa3990aa712aa3ec6681dfafd50def3a0 4814fce142b95b910d0cbe87c3c304b15f279a0a 8c1c8d51a82b07712c8e64109717f615a5e4a81e 100644 ded65fd3b11aa6a0b82f6ab170eaee5411c5ce2e 1 pom.xml 100644 29418c82b95793ee6b688bf6761ee53d8f42c1c5 2 pom.xml 100644 9bc48d5ca4b62d32cebf09f678148c8ef35b0820 3 pom.xml

Auto-merging pom.xml CONFLICT (content): Merge conflict in pom.xml ```

**Expected Behaviour**: merge driver is executed

Proof merge driver is really active (Note: _ort_ doesn't respect `HEAD:.gitattributes`, but _recursive_ does)

```shell $ git merge -s recursive -m "Automatic merge" --no-ff --no-log --allow-unrelated-histories --no-verify 561da86fa3990aa712aa3ec6681dfafd50def3a0 4814fce142b95b910d0cbe87c3c304b15f279a0a

Merge Driver is active

Auto-merging pom.xml Merge made by the 'recursive' strategy. ```

Use "new" default merge strategy _ort_. Changed with Git 2.34 (breaks backwards compatibility). _ort_ **doesn't** execute the merge driver

```shell $ git merge -m 'Automatic merge' --no-ff --no-log --allow-unrelated-histories --no-verify 561da86fa3990aa712aa3ec6681dfafd50def3a0 4814fce142b95b910d0cbe87c3c304b15f279a0a Auto-merging pom.xml CONFLICT (content): Merge conflict in pom.xml Automatic merge failed; fix conflicts and then commit the result. ``` ------------------------------------------------------------------------------------------------------------------------

Kind Regards, André

Diese E-Mail enthält vertrauliche und/oder rechtlich geschützte Informationen. Wenn Sie nicht der richtige Adressat sind oder diese E-Mail irrtümlich erhalten haben, informieren Sie bitte sofort den Absender und vernichten Sie diese E-Mail. Das unerlaubte Kopieren sowie die unbefugte Weitergabe dieser Mail ist nicht gestattet. This e-mail may contain confidential and/or privileged information. If you are not the intended recipient (or have received this e-mail in error) please notify the sender immediately and destroy this e-mail. Any unauthorized copying, disclosure or distribution of the material in this e-mail is strictly forbidden.