Restore main into develop's lineage

main carries one commit develop does not: the merge GitHub created when
develop went in for v2.0.0. Both of its parents are already ancestors
here and it contributes no file, so it is a marker, not content — but it
left main outside develop's history, and a fast-forward of main to the
release would be rejected.

Recorded with -s ours: develop's tree is unchanged, main becomes an
ancestor, and main can fast-forward to the release again.

Author: DemchaAV