The interface into "xdiff" library used to discover the offset and
size of a generated patch hunk by first formatting it into the
textual hunk header "@@ -n,m +k,l @@" and then parsing the numbers
out.  A new interface has been introduced to allow callers a more
direct access to them.

* jk/xdiff-interface:
  xdiff-interface: drop parse_hunk_header()
  range-diff: use a hunk callback
  diff: convert --check to use a hunk callback
  combine-diff: use an xdiff hunk callback
  diff: use hunk callback for word-diff
  diff: discard hunk headers for patch-ids earlier
  diff: avoid generating unused hunk header lines
  xdiff-interface: provide a separate consume callback for hunks
  xdiff: provide a separate emit callback for hunks