Git: Open merge tool without conflict

Question

I have problems with a merge, but git doesn't mark the file as conflicted. When I try to open the file with git mergetool, git just says: No files need merging.

How can I open a file without merge conflict in a three way compare?

Answer 1

You can do the following:

git merge --no-commit topic
git checkout --merge that_file
git mergetool that_file
git commit

That is, you avoid that the successful merge creates a commit, then you pretend that there was a conflict in that_file, then you can treat the file with git mergetool. Finally, you commit the result.

Answer 2

sadly I coudln't reproduce @Filp Stefanov's success, so I did it the hard way by hand

(from the repository root)

git merge --no-ff --no-commit <topic>
git show HEAD:<file> > <file>.ours
git show MERGE_HEAD:<file> > <file>.theirs
$(git config merge.tool) <file>.ours <file> <file>.theirs
rm <file>.ours <file>.theirs
git add <file>
git commit

said outloud thats,

1. show our version of the file to stdout and save that somewhere
2. show their of the file to stdout and save that somewhere
3. open the merge tool of ours, current, theirs
4. do your merge
5. delete the .ours/.theirs files
6. add & commit the merged file

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I wouldn't be surprised if that's basically what git does natively