Truncate filenames in the leadning end in pg_basebackup verbose output

When truncating at the end, like before, the output would often end up
just showing the path instead of the filename.

Also increase the length of the filename by 5, which still keeps us at
less than 80 characters in most outputs.
Make size-output fixed length in pg_basebackup verbose mode

This way the line doesn't shift right as the amount of data processed
increases.