xdiff: provide a separate emit callback for hunks

xdiff: provide a separate emit callback for hunks

The xdiff library always emits hunk header lines to our callbacks as
formatted strings like "@@ -a,b +c,d @@\n". This is convenient if we're
going to output a diff, but less so if we actually need to compute using
those numbers, which requires re-parsing the line.

In preparation for moving away from this, let's teach xdiff a new
callback function which gets the broken-out hunk information. To help
callers that don't want to use this new callback, if it's NULL we'll
continue to format the hunk header into a string.

Note that this function renames the "outf" callback to "out_line", as
well. This isn't strictly necessary, but helps in two ways:

  1. Now that there are two callbacks, it's nice to use more descriptive
     names.

  2. Many callers did not zero the emit_callback_data struct, and needed
     to be modified to set ecb.out_hunk to NULL. By changing the name of
     the existing struct member, that guarantees that any new callers
     from in-flight topics will break the build and be examined
     manually.

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