diff --color-moved: call comparison function directly

diff --color-moved: call comparison function directly

This change will allow us to easily combine pmb_advance_or_null() and
pmb_advance_or_null_multi_match() in the next commit. Calling
xdiff_compare_lines() directly rather than using a function pointer
from the hash map has little effect on the run time.

Test                                                                  HEAD^             HEAD
-------------------------------------------------------------------------------------------------------------
4002.1: diff --no-color-moved --no-color-moved-ws large change        0.38(0.35+0.03)   0.38(0.32+0.06) +0.0%
4002.2: diff --color-moved --no-color-moved-ws large change           0.87(0.83+0.04)   0.87(0.80+0.06) +0.0%
4002.3: diff --color-moved-ws=allow-indentation-change large change   0.97(0.92+0.04)   0.97(0.93+0.04) +0.0%
4002.4: log --no-color-moved --no-color-moved-ws                      1.17(1.06+0.10)   1.16(1.10+0.05) -0.9%
4002.5: log --color-moved --no-color-moved-ws                         1.32(1.24+0.08)   1.31(1.22+0.09) -0.8%
4002.6: log --color-moved-ws=allow-indentation-change                 1.36(1.25+0.10)   1.35(1.25+0.10) -0.7%

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