xdiff: avoid unnecessary memory allocations

xdiff: avoid unnecessary memory allocations

rindex and ha are only used by xdl_cleanup_records() which is not
called by the histogram or patience algorithms. The perf test results
show a small reduction in run time but that is probably within the
noise.

Test                                  HEAD^             HEAD
-----------------------------------------------------------------------------
4000.1: log -3000 (baseline)          0.19(0.17+0.02)   0.19(0.12+0.07) +0.0%
4000.2: log --raw -3000 (tree-only)   0.98(0.78+0.20)   0.98(0.81+0.16) +0.0%
4000.3: log -p -3000 (Myers)          4.81(4.15+0.64)   4.81(4.23+0.56) +0.0%
4000.4: log -p -3000 --histogram      5.87(5.19+0.66)   5.83(5.11+0.70) -0.7%
4000.5: log -p -3000 --patience       5.35(4.60+0.73)   5.31(4.61+0.69) -0.7%

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