blame: Revert common subtree elimination "optimization"

blame: Revert common subtree elimination "optimization"

This partially reverts 6de12836d72fe4cba9afc297c8988c6fff851fa9.

Performing a TreeWalk over 2 trees to identify and skip unmodified
subtrees to pass all blame onto an ancestor appears to be a micro
optimization that works for a very limited number of files.  In the
general case the 2 tree walk is slowing down blame more than it helps
to speed it up.

I keep coming up with files in multiple repositories where 6de128 is
making things worse, not better, and only one example where it
actually improved performance, render_view_impl.cc in chromium
as described in the commit message.

Change-Id: Ic6d5fff22acb5ab6485614a07bdb388e8c336679