showlinenum : Parse diff line indicators

- Parse diff line indicators [ +-]

- Use padding instead of a pseudo line number for removed lines.

Prior to this change the script would show where in the new/child file a
removed line would have been removed from but I found that to be rather
confusing. There was also a bug because the pseudo line number was
behind by 1 when the new file's hunk was count 0 (empty hunk).

Now there is space equal to the number of digits in the line number.

Before:
showlinenum.awk:303:+    else
showlinenum.awk:304:     {
showlinenum.awk:305:-        line++;

After:
showlinenum.awk:303:+    else
showlinenum.awk:304:     {
showlinenum.awk:   :-        line++;