gitweb: make leftmost column of blame less cluttered.

gitweb: make leftmost column of blame less cluttered.

Instead of labelling each and every line with clickable commit
object name, this makes the blame output to show them only on
the first line of each group of lines from the same revision.
Placing too many lines in one group would make the commit object
name to appear too widely separated and also makes it consume
more memory, the number of lines in one group is capped to 20
lines or so.

Also it makes mouse-over to show the minimum authorship and
authordate information for extra cuteness ;-).

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