Add a menu item to invoke full copy detection in blame.

Add a menu item to invoke full copy detection in blame.

Add a context menu item to invoke blame -C -C -C on a chunk
of the file. The results are used to update the 'original
location' column of the blame display.

The chunk is computed as the smallest line range that covers
both the 'last change' and 'original location' ranges of the
line that was clicked to open the menu.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>