xdiff: simplify comparison

xdiff: simplify comparison

Now that the histogram algorithm calls xdl_classify_record() it is no
longer necessary to use xdl_recmatch() to compare lines, it is
sufficient just to compare the hash values. This has a negligible
effect on performance.

Test                                  HEAD~1            HEAD
-----------------------------------------------------------------------------
4000.1: log -3000 (baseline)          0.19(0.12+0.07)   0.18(0.14+0.04) -5.3%
4000.2: log --raw -3000 (tree-only)   0.98(0.81+0.16)   0.98(0.79+0.18) +0.0%
4000.3: log -p -3000 (Myers)          4.81(4.23+0.56)   4.80(4.26+0.53) -0.2%
4000.4: log -p -3000 --histogram      5.83(5.11+0.70)   5.82(5.15+0.65) -0.2%
4000.5: log -p -3000 --patience       5.31(4.61+0.69)   5.30(4.54+0.75) -0.2%

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>