blame: -b (blame.blankboundary) and --root (blame.showroot)

blame: -b (blame.blankboundary) and --root (blame.showroot)

When blame.blankboundary is set (or -b option is given), commit
object names are blanked out in the "human readable" output
format for boundary commits.

When blame.showroot is not set (or --root is not given), the
root commits are treated as boundary commits.  The code still
attributes the lines to them, but with -b their object names are
not shown.

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