xdiff: rename "struct group" to "struct xdlgroup"

xdiff: rename "struct group" to "struct xdlgroup"

Commit e8adf23 (xdl_change_compact(): introduce the concept
of a change group, 2016-08-22) added a "struct group" type
to xdiff/xdiffi.c. But the POSIX system header "grp.h"
already defines "struct group" (it is part of the getgrnam
interface). This happens to work because the new type is
local to xdiffi.c, and the xdiff code includes a relatively
small set of system headers. But it will break compilation
if xdiff ever switches to using git-compat-util.h.  It can
also probably cause confusion with tools that look at the
whole code base, like coccinelle or ctags.

Let's resolve by giving the xdiff variant a scoped name,
which is closer to other xdiff types anyway (e.g.,
xdlfile_t, though note that xdiff is fond if typedefs when
Git usually is not).

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