shortlog: take the first populated line of the description

shortlog: take the first populated line of the description

Way back the perl version of shortlog would take the first populated line
of the commit body.  The builtin version mearly takes the first line.
This leads to empty shortlog entries when there is some viable text in
the commit.

Reinstate this behaviour igoring all lines with nothing but whitespace.
This is often useful when dealing with commits imported from foreign SCMs
that do not tidy up the log message of useless blank lines at the beginning.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>