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.