treewide: convert users of `whence` to the new source field

treewide: convert users of `whence` to the new source field

The `whence` field has become redundant now that callers can learn about
the exact source an object has been looked up from via the `struct
object_info_source::source` field.

Adapt callers to use the new field. Note that all callsites already set
up the `info.sourcep` request pointer, so the conversion is rather
straight-forward.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>