pretty: make the skip_blank_lines() function public

This function will be used also in the find_commit_subject()
function.

While at it, rename the function to reflect that it skips not only
empty lines, but any lines consisting of only whitespace, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>