reset.c: finish entire cmd_reset() whether or not pathspec is given

reset.c: finish entire cmd_reset() whether or not pathspec is given

By not returning from inside the "if (pathspec)" block, we can let the
pathspec-aware and pathspec-less code share a bit more, making it
easier to make future changes that should affect both cases. This also
highlights the similarity between read_from_tree() and reset_index().

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