git-gui: Display original filename and line number in blame.

git-gui: Display original filename and line number in blame.

When we annotate a file and show its line data, we're already asking
for copy and movement detection (-M -C).  This costs extra time, but
gives extra data.  Since we are asking for the extra data we really
should show it to the user.

Now the blame UI has two additional columns, one for the original
filename (in the case of a move/copy between files) and one for the
original line number of the current line of code.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>