blame: Micro optimize blob lookup in tree

blame: Micro optimize blob lookup in tree

Avoid converting the raw mode to FileMode. This is an expensive
if-else-if sort of test to just check if the thing is a blob.
Instead test the bit mask directly, which is at least a few
instructions shorter.

The TreeWalk is already recursive and will auto-dive into any
subtrees found. isSubtree check is unnecessary, as is the loop,
as only one result will ever be returned by next().

Change-Id: I9fb25229ebed857469427bfbdf74aedebfddfac8