xdiff-interface: hide the whole "xdiff_emit_state" business from the caller

xdiff-interface: hide the whole "xdiff_emit_state" business from the caller

This further enhances xdi_diff_outf() interface so that it takes two
common parameters: the callback function that processes one line at a
time, and a pointer to its application specific callback data structure.
xdi_diff_outf() creates its own "xdiff_emit_state" structure and stashes
these two away inside it, which is used by the lowest level output
function in the xdiff_outf() callchain, consume_one(), to call back to the
application layer.  With this restructuring, we lift the requirement that
the caller supplied callback data structure embeds xdiff_emit_state
structure as its first member.

Signed-off-by: Junio C Hamano <gitster@pobox.com>