claw back annexed file scan speedup

claw back annexed file scan speedup

Following commit c941ab6f5b98becfa2880cb69cbc2c4a39057cd9, this avoids
the second, redundant scan when annex.thin is not set.

The benchmark now runs in 35.5 seconds, down from 40 seconds.

Note that the inode cache of the annex object has to be passed to
addInodeCaches now, because it might not already be in the inode caches,
unlike previously.

Sponsored-by: Dartmouth College's Datalad project