Alex Henrie [off-list ref] writes:

Changes from v5:
- Add commit message note about --no-rebase-merges having always worked
- Add commit message note about the test for --no-rebase-merges being
  somewhat contrived
- Rephrase the documentation to avoid using the phrase "By default" with
  two different meanings, and in so doing clarify that --rebase-merges
  without an argument is not the same as --no-rebase-merges and not
  passing --rebase-merges is not the same as passing
  --rebase-merges=no-rebase-cousins
- Add commit message note about keeping --rebase-merges="" for now out
  of an abundance of caution
- Rephrase the warning about --rebase-merges="" to recommend
  --rebase-merges without an argument instead, and clarify that that
  will do the same thing
- Remove the test for --rebase-merges=""
- Rename the config option from rebase.merges to rebase.rebaseMerges and
  explain why in the commit message
- Add commit message note about why "true" is a valid option for
  rebase.rebaseMerges and why --rebase-merges without an argument does
  not clobber the mode specified in rebase.rebaseMerges
- Rephrase the documentation to clarify that --rebase-merges without an
  argument does not clobber the mode specified in rebase.rebaseMerges
- Add another test for incompatible options

This version addresses all of the concerns I had with the previous
version. Thanks!

Besides the concerns that other reviewers raised and a possible
mechanical error, I don't have any outstanding concerns. I'd be happy
to see this merged when those are resolved :)