combine-diff: factor out stat-format mask

combine-diff: factor out stat-format mask

There are several conditionals in the combine diff code that check if
we're doing --stat or --numstat output. Since these must all remain in
sync, let's pull them out into a separate bit-mask.

Arguably this could go into diff.h along with the other DIFF_FORMAT
macros, but it's not clear that the definition of "which formats are
stat" is universal (e.g., does --dirstat count? --summary?). So let's
keep this local to combine-diff.c, where the meaning is more clearly
"stat formats that combine-diff supports".

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>