We use Tortoise in concert with (the brilliant) Extensions because
some of our developers appreciate the icon overlays in Explorer, but
they go back to Extensions for git functions. One exception is
interactive rebasing: in Tortoise Git you shift-select a range of
commits and "Combine to one commit" from the right-click menu. It
could not be easier.
There is nothing similar in Extensions.
"Maniplate Commit -> Squash (or fixup) commit" just takes me to the
regular commit dialogue with no files to commit.
You're probably already aware of this, but if someone is looking for
an area where Git Extensions can catch up to Tortoise Git, here you
go.