SizeDeltaGcEstimation should compute the repository size from the same source

SizeDeltaGcEstimation consumes two sources for the computation of the repository size. When the estimation algorithm is run, the current repository size is the approximated size provided by FileStoreStats, while the previous repository size, taken from gc.log, is the exact size computed by TarFiles#size. SizeDeltaGcEstimation should use always the same source for the size of the repository.