stats: Expose absolute and relative paths for alternates

Currently the Git alternates file parsing done with stats.AlternatesInfoForRepository() always converts the alternates directory paths to be absolute. This is problematic because sometimes it is required to see what the exact path of the alternates recorded in the alternates file is.

This change refactors stats.AlternatesInfoForRepository() to instead return the raw directory paths contained within a repository's Git alternates file. AbsoluteObjectDirectories() has been added to convert to absolute paths if needed.