When reading conflicts:
1. Add a `type` field. `text` works as before, and has `sections`;
`text-editor` is a file with ambiguous conflict markers that can only
be resolved in an editor.
2. Add a `content_path` field pointing to a JSON representation of the
file's content for a single file.
3. Hitting `content_path` returns a similar datastructure to the `file`,
but without the `content_path` and `sections` fields, and with a
`content` field containing the full contents of the file (with
conflict markers).
When writing conflicts:
1. Instead of `sections` being at the top level, they are now in a
`files` array. This matches the read format better.
2. The `files` array contains file hashes, each of which must contain:
a. `new_path`
b. `old_path`
c. EITHER `sections` (which works as before) or `content` (with the
full content of the resolved file).
- Add match line header to expected result for `File#sections`.
- Lowercase CSS colours.
- Remove unused `diff_refs` keyword argument.
- Rename `parent` -> `parent_file`, to be more explicit.
- Skip an iteration when highlighting.
