The internet tells me about git-imerge:
Lets you test every intermediate state. If there is a problem, you can use "git bisect" to find the exact pairwise merge that was faulty. You can redo that merge and continue the incremental merge from there (retaining earlier pairwise merges).
But how can I do that?
To start the operation of eg. merging branch newFeature to master, I would use
git checkout master
git-imerge start --name=mergeMyNewFeatureToMaster --first-parent --goal=merge newFeature
And then resolve all the conflicts that imerge finds by itself. After finishing, I see that somewhere a bug has been inserted. I search for it using
git bisect start
git bisect good ...
git bisect bad ...
...
But when I have found the merge commit that introduced the problem – how can I redo that merge and continue the incremental merge from there?