xdiff: reduce size of action arrays

xdiff: reduce size of action arrays

When the myers algorithm is selected the input files are pre-processed
to remove any common prefix and suffix. Then any lines that appear
only in one side of the diff are marked as changed and frequently
occurring lines are marked as changed if they are adjacent to a
changed line. This step requires a couple of temporary arrays. As as
the common prefix and suffix have already been removed, the arrays
only need to be big enough to hold the lines between them, not the
whole file. Reduce the size of the arrays and adjust the loops that
use them accordingly while taking care to keep indexing the arrays
in xdfile_t with absolute line numbers.

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