R1:a82a2b84599d

Simplify non-bare working copy rules for the new "git fetch" strategy

Summary:
Ref T13280. In D20421, I changed our observe strategy to `git fetch <uri>` in all cases.

This doesn't work in an ancient, non-bare repository if `master` is checked out and `master` is also fetch: `git` refuses to overwrite the local ref unless we pass `--update-head-ok`. Pass this flag.

Also, remove some code which examines branches and tags in a special way for non-bare working copies. The old `git fetch <origin>` code…