reset.c: pass pathspec around instead of (prefix, argv) pair

reset.c: pass pathspec around instead of (prefix, argv) pair

We use the path arguments in two places in reset.c: in
interactive_reset() and read_from_tree(). Both of these call
get_pathspec(), so we pass the (prefix, argv) pair to both
functions. Move the call to get_pathspec() out of these methods, for
two reasons: 1) One argument is simpler than two. 2) It lets us use
the (arguably clearer) "if (pathspec)" in place of "if (i < argc)".

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>