Regexp specifying whether to set fill prefix from a one-line paragraph.
When a paragraph has just one line, then after `adaptive-fill-regexp'
finds the prefix at the beginning of the line, if it doesn't
match this regexp, it is replaced with whitespace.
By default, this regexp matches sequences of just spaces and tabs.
However, we never use a prefix from a one-line paragraph
if it would act as a paragraph-starter on the second line.