Multiple fetches when unshallowing a shallow clone

Hi all,

Would anyone be willing to help me understand some shallow-clone
behavior?  (I found a bug in Dulwich, and I'm looking for some context
so I can determine how to fix it.)

I learned that cgit sometimes performs two fetches for a `git fetch
--unshallow`: one with depth 'infinity', and a subsequent one with
depth zero.

Could anyone answer:
1) What is the purpose of the second fetch?
2) What does this depth of zero mean? Is it the same as a depth of
infinity?  (I assume not... but, since I thought the smallest
meaningful depth was 1, I don't know what else it might mean.)

Thank you!
Jason