Support per-file modification dates

[begriffs]

Back in 01aa024 the server would revwalk to find which ancestral commit introduced the blob for each tree entry, and used that commit time as modification time for the file. Now we list all files with the modification time of the master commit itself.

Could try revisiting this and doing a single more efficient revwalk checking for all tree entries at once. It would still be slower than the current method, but remains to be seen how slow.