merge - Is there a way to make Git mark a file as conflicted?

Question

It's possible to commit files that contains conflict data. Is there a way to mark these files as conflicted again, so that running git mergetool will generate the necessary files and run the merge tool?

Answer 1

If the index is already in a conflict state, simply check out the file with the --conflict=merge flag:

git checkout --conflict=merge file

If the index is clean because the unresolved file has been [erroneously] added, just reset it before checking it out:

git reset file
git checkout --conflict=merge file

This will allow you to resume conflict resolution normally (e.g., git mergetool).

NOTE: Promoting a comment to @jakub-nar?bski's answer into its own answer by request from @fourpastmidnight. :)