Add --log-size to git log to print message size

Add --log-size to git log to print message size

With this option git-log prints log message size
just before the corresponding message.

Porcelain tools could use this to speedup parsing
of git-log output.

Note that size refers to log message only. If also
patch content is shown its size is not included.

In case it is not possible to know the size upfront
size value is set to zero.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>