Fix bug with 'git remote set-branches origin main'

[jansel]

I was hitting a bug where every PR would use the same name jansel/stack/1, for example:

$ stack-pr view
VIEW
Stack:
   * 87de0037 (no PR, 'jansel/stack/1' -> 'jansel/stack/1'): ...
   * 683f11a4 (#81, 'jansel/stack/1' -> 'main'): ...

jansel/stack/2 would never get used, and stack-pr would clobber itself and be very broken.

The issue was my git was configured to only fetch main rather than all branches. You can repro this with:

git remote set-branches origin main

(and also delete all your local cached remote refs).

One fix would be:

git remote set-branches origin '*'

however, this modifies my global git config -- and users won't know they need to run that command.

This PR fixes the issue by forcing all branches to be fetched.

One potential improvement would be a more narrow fetch like:

+refs/heads/jansel/stack/*:refs/remotes/origin/jansel/stack/*

which will make stack-pr faster in large multi-user projects, though I will leave that to others to implement since I don't have a great testing setup.