Change the methodology for handling indent

Now, indent is left alone until after lines have been split into fields.
We then find the common indent remaining in the 1st field of each line
that will be affected by the align operation (for GTabularize,
non-matching lines don't factor in here).  Also, blank lines don't
affect the calculation of longest common indent.

Before calculation of field widths, leading indent is stripped from all
fields, and after the fields have been joined back together we re-add
the common indent that was stripped earlier.