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>

builtin-merge.c

@@ -361,7 +361,7 @@ static void merge_name(const char *remote, struct strbuf *msg)
 	int len, early;
 
 	len = strlen(remote);
-	if (interpret_nth_last_branch(remote, &bname) == len)
+	if (interpret_branch_name(remote, &bname) == len)
 		remote = bname.buf;
 
 	memset(branch_head, 0, sizeof(branch_head));