Basic "subtree merge --squash" support.

Basic "subtree merge --squash" support.

Instead of merging in the history of the entire subproject, just squash it
all into one commit, but try to at least track which commits we used so that
we can do future merges correctly.

Bonus feature: we can actually switch branches of the subproject this way,
just by "squash merging" back and forth from one tag to another.