Extend `replace` command to output file diffs

Created by: courier-new

Currently, the compute replace command only outputs the changed file contents.

For search-and-replace batch changes, we would like to implement support for an alternative output of the file diff when a new command, replace.diff(...), is used. This will enable us to leverage this command for the UI previews and ChangesetSpec. We should ensure we're not creating 2 requests to gitserver for the same file this way.

There are currently 3 types of Compute result: Text, TextExtra, and MatchContext. replace.diff(...) should output a 4th type of result like ReplaceDiff that also contains all the necessary context about where the diff was generated:

• the repository ID
• the base ref
• the base revision

This issue has as a prerequisite that we are able to generate patches in Go without calling out to git diff (https://github.com/sourcegraph/sourcegraph/issues/39194).