gitweb: use blame --porcelain

gitweb: use blame --porcelain

This makes gitweb (git_blame2) use "blame --porcelain", which
lets the caller to figure out which line in the original version
each line comes from.  Using this information, change the
behaviour of clicking the line number to go to the line of the
blame output for the original commit.

Before, clicking the line number meant "scoll up to show this
line at the beginning of the page", which was not all that
useful.  The new behaviour lets you click on the line you are
interested in to view the line in the context it was introduced,
and keep digging deeper as you examine it.

Signed-off-by: Junio C Hamano <junkio@cox.net>