How to debug a "git merge"?

Hi,

I am investigating a Git merge (a86dd40fe) in which an older version of a file won over the newer version. I try to understand why this is the case. I can reproduce the merge with the following commands: $ git checkout -b test a02fa3303 $ GIT_MERGE_VERBOSITY=5 git merge --verbose c1b82995c

The merge actually generates a merge conflict but not for my problematic file. The common ancestor of the two parents (merge base) is b91161554.

The merge graph is not pretty (the committers don't have a clean branching scheme) but I cannot spot a problem between the merge commit and the common ancestor: $ git log --graph --oneline a86dd40fe

Can you give me a hint how to debug this merge further? How can I understand why Git picked a certain version of a file in a merge?

I am using Git 2.16.2 on Linux.

Thanks, Lars