> if I understand correctly then Fossil's migration is straightforward because they did not address the same issues Git chose to.

I think more is at play here.

(1) You can set Fossil to ignore all SHA1 artifacts using the "shun-sha1" hash policy.

(2) The excess complication in the Git migration strategy is likely due to the inability of the underlying Git file formats to handle two different hash algorithms in the same repository at the same time.

But, I could be wrong. Post a rebuttal if you have evidence to the contrary.