git-blame: show lines attributed to boundary commits differently.

git-blame: show lines attributed to boundary commits differently.

When blaming with revision ranges, often many lines are attributed
to different commits at the boundary, but they are not interesting
for the purpose of finding project history during that revision range.

This outputs the lines blamed on boundary commits differently. When
showing "human format" output, their SHA-1 are shown with '^' prefixed.
In "porcelain format", the commit will be shown with an extra attribute
line "boundary".

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