So instead of

     # This is the combination of 4 commits
     # This is the first commit message
     Base subject

     Base body

     # This is the second commit message
     # Another subject

     # Another body

     # This is the third commit message
     # fixup! Base subject

     # This is the fourth commit message
     # amend! Another subject
     A better subject

     A better body

We'd have

     # This is the combination of 4 commits
     # 123 Base subject
     # 456 Another subject
     # 789 fixup! Base subject
     # abc amend! Another subject

     Base Subject

     Base Body

     Another subject

     Another Body

I think this makes it a lot harder to read. If every commit body was
always just a single paragraph it could make sense, but it's generally
not. Look at the commits in this series, with no delimiter of where
one commit message ends and the next one starts, it would be very
confusing.

It would be good to error out if the user tries squash a fixup! style
commit and range does not contain its target commit.

Good point, I don't see a good reason to allow this.

There does seem to be some support for merges in this patch series which
I think behaves pretty sensibly. If we have

           C - D
          /     \
   - A - B - E - F - G

Then squashing A..G should be fine because the parents of F are in the
range and it looks like we support that. Squashing should B..G without
--ancestry-path should be safe as well because B ends up as the parent
of the squashed commit but we don't have a way to disable
--ancestry-path (and maybe we don't want to add one). Squashing F^@..G
might be useful to fixup a merge (though perhaps amending F rather than
creating G is a simpler way to fix a broken merge). Squashing E..G does
not make sense because the range does not include one of the merge parents.

Thanks for a good explanation here!


Harald