annotate: use argv_array

Simplify the code and get rid of some magic constants by using
argv_array to build the argument list for cmd_blame.  Be lazy and let
the OS release our allocated memory, as before.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>