blame and friends: adjust to multiple pathspec change.

blame and friends: adjust to multiple pathspec change.

This makes things that include revision.h build again.

Blame is also built, but I am not sure how well it works (or how
well it worked to begin with) -- it was relying on tree-diff to
be using whatever pathspec was used the last time, which smells
a bit suspicious.

Signed-off-by: Junio C Hamano <junkio@cox.net>