Display styled unified diffs

The unified diffs are shown in a two-column table, giving us a line
cursor position in the first column and a pre-formatted hunk text in
the second column.  Styles are applied based upon the rules used in
a standard two-way diff, color-coding the lines to make it easier to
read what is displayed quickly.

Signed-off-by: Shawn O. Pearce <sop@google.com>
12 files changed