git-blame: --show-number (and -n)

git-blame: --show-number (and -n)

The new option makes the command's native output format show the
original line number in the blamed revision.

Note: the current implementation of find_orig_linenum involves
linear search through the line_map array every time.  It should
probably build a reverse map upfront and do a simple look-up to
speed things up, but I'll leave it to more clever and beautiful
people ;-).

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