blame: use xdi_diff_hunks(), get rid of struct patch

blame: use xdi_diff_hunks(), get rid of struct patch

Based on a patch by Brian Downing, this replaces the struct patch based
code for blame passing with calls to xdi_diff_hunks().  This way we
avoid generating and then parsing patches; we only let the interesting
infos be passed to our callbacks instead.  This makes blame a bit faster:

   $ blame="./git blame -M -C -C -p --incremental v1.6.0"

   # master
   $ /usr/bin/time $blame Makefile >/dev/null
   1.38user 0.14system 0:01.52elapsed 100%CPU (0avgtext+0avgdata 0maxresident)k
   0inputs+0outputs (0major+12226minor)pagefaults 0swaps
   $ /usr/bin/time $blame cache.h >/dev/null
   1.66user 0.13system 0:01.80elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
   0inputs+0outputs (0major+12262minor)pagefaults 0swaps

   # this patch series
   $ /usr/bin/time $blame Makefile >/dev/null
   1.27user 0.12system 0:01.40elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
   0inputs+0outputs (0major+11836minor)pagefaults 0swaps
   $ /usr/bin/time $blame cache.h >/dev/null
   1.52user 0.12system 0:01.70elapsed 97%CPU (0avgtext+0avgdata 0maxresident)k
   0inputs+0outputs (0major+12052minor)pagefaults 0swaps

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>