R1:20892ae50236

Simplify "git fetch" behavior in the Pull daemon

Summary:
Ref T12392. The logic currently goes like this:

- Try a fetch.
- If that fails, try repairing the origin URI.
- Then try again.

This is pretty complicated, and we can use this simpler logic instead:

- Set the origin URI to the right value.
- Try a fetch.

Setting the origin URI is very fast. This can normally only get us in any trouble in very obscure situations which haven't occurred for many years:

- Pretty much all of this is…