Rename interpret/substitute nth_last_branch functions

Rename interpret/substitute nth_last_branch functions

These allow you to say "git checkout @{-2}" to switch to the branch two
"branch switching" ago by pretending as if you typed the name of that
branch.  As it is likely that we will be introducing more short-hands to
write the name of a branch without writing it explicitly, rename the
functions from "nth_last_branch" to more generic "branch_name", to prepare
for different semantics.

Signed-off-by: Junio C Hamano <gitster@pobox.com>