diff: export diffstat interface

diff: export diffstat interface

Make the diffstat interface (namely, the diffstat_t struct and
compute_diffstat) no longer be internal to diff.c and allow it to be used
by other parts of git.

This is helpful for code that may want to easily extract information
from files using the diff machinery, while flushing it differently from
how the show_* functions used by diff_flush() do it. One example is the
builtin implementation of git-add--interactive's status.

Signed-off-by: Daniel Ferreira <bnmvco@gmail.com>
Signed-off-by: Slavica Đukić <slawica92@hotmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>