All call paths already check before calling the function that the
condition is true. One caller - `tree::try_resolve_file_conflict()` -
checks it itself. The other caller -
`conflicts::materialize_merge_result()` - doesn't, but its callers
have checked it via `extract_file_conflict_as_single_hunk()`.
The deleted comment about empty strings seems to be obsolete since
e48ace56d1e9. The caller pads the inputs with empty strings since that
commit.
I think we should ideally change this function's signature to make it
impossible to call it with bad inputs, and I hope to get back to that
soon.
1ec906b114