nofib-analyse: Support comparing multiple runs in the summary table

and adjust the table layout to have at least one space padding, even
with long input file names.
nofib-analyize: Include % in LaTeX output

I see no reason why this should be different from the text output,
especially as it is important to distinguish absolute from relative
values in the runtime column. It is also easier to remove the % (using
search-and-replace) than to add it.