Allow alternate "low-level" emit function from xdl_diff

Allow alternate "low-level" emit function from xdl_diff

For some users (e.g. git blame), getting textual patch output is just
extra work, as they can get all the information they need from the low-
level diff structures.  Allow for an alternate low-level emit function
to be defined to allow bypassing the textual patch generation; set
xemitconf_t's emit_func member to enable this.

The (void (*)()) type is pretty ugly, but the alternative would be to
include most of the private xdiff headers in xdiff.h to get the types
required for the "proper" function prototype.  Also, a (void *) won't
work, as ANSI C doesn't allow a function pointer to be cast to an
object pointer.

Signed-off-by: Brian Downing <bdowning@lavos.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>