xdiff: optimise for no whitespace difference when ignoring whitespace.

xdiff: optimise for no whitespace difference when ignoring whitespace.

In xdl_recmatch, do the memcmp to check if the two lines are equal before
checking if whitespace flags are set.  If the lines are identical, then
there is no need to check if they differ only in whitespace.
This makes the common case (there is no whitespace difference) faster.
It costs the case where lines are the same length and contain
whitespace differences, but the common case is more than 20% faster.

Signed-off-by: Dylan Reid <dgreid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>