builtin-blame.c: move prepare_final() into a separate function.

builtin-blame.c: move prepare_final() into a separate function.

After parsing the command line, we have a long loop to compute the commit
object to start annotating from.  Move the logic to a separate function,
so that later patches become easier to read.

It also makes fill_origin_blob() return void; the check is always done
on !file->ptr, and nobody looks at the return value from the function.

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