git/objectpool: Convert usage of `FullPath()` to `Path()`

Object pools provide two ways to calculate the path of a pool: FullPath() returns the path without verifying whether the pool exists, while Path() verifies that the target exists. The latter is what we already use in the context of the localrepo.Repo package, and it gives us more safeguards to ensure that the object pool really exists when it ought to exist.

Refactor our codebase to consistently use Path() and remove the FullPath() function.