corrupt the created indexes.

row_merge(): Make "half" an in/out parameter. Determine the offset of
half the output file. Copy the last blocks record-by-record instead of
block-by-block, so that the records can be counted. Check that the
input and output have matching n_rec.

row_merge_sort(): Do not assume that two blocks of size N are merged
into a block of size 2*N. The output block can be shorter than the
input if the last page of each input block is almost empty. Use an
accurate termination condition, based on the "half" computed by
row_merge().

row_merge_read(), row_merge_write(), row_merge_blocks(): Add debug output.

merge_file_t, row_merge_file_create(): Add n_rec, the number of records
in the merge file.

row_merge_read_clustered_index(): Update n_rec.

row_merge_blocks(): Update and check n_rec.

row_merge_blocks_copy(): New function, for copying the last blocks in
row_merge().  Update and check n_rec.

This bug was discovered with a user-supplied test case that creates an
index where the initial temporary file is 249 one-megabyte blocks and
the merged files become smaller. In the test, possible merge record
sizes are 10, 18, and 26 bytes.

rb://150 approved by Sunny Bains.  This addresses Issue #320.