Add UnifiedDiffWriter.write_index, for writing Index: lines.
Review Request #12629 — Created Sept. 23, 2022 and submitted
UnifiedDiffWriter.write_index(), which writes a standard
Index: ...line, followed by 67
This is a fairly standard convention used by Unified Diff variants to
help separate files when there may not otherwise be a clear boundary.
For instance, when there may be metadata changes or binary file
differences, but no
Like other methods, this accepts byte strings or Unicode strings,
handling encoding automatically.
Unit tests passed.
Used this in some in-progress work.