Fix tense of test case heading #cosmetic #writing
Better merged Main tree for assim'd merges in impt. case

The code itself is just 3 new lines and 1 changed line, but also add a
nice big comment describing why it works and the important use case
where it does exactly the right thing.

Also add a test case closely matching the description of the common use
case, and importantly, a negative test case where we still want to use
HEAD.
Add note about synthetic merges in README

I feel good enough about this design decision to document it, but
leaving room for it to change.
Add TODO to README about cherry-pick-like commits