git-gui: Refactored diff line display formatting logic.

git-gui: Refactored diff line display formatting logic.

The tags used for diff formatting (which I inherited from gitool) just
didn't make a whole lot of sense, especially if you wanted to try to
match them to the diff output you were seeing on screen.  It did not
help that the diff-index -c output's first two columns are also munged
to make the diff output more user friendly.

So this is a large refactoring of the tags used for diff display.  Now
our tag names match what we put in the left column of each line, which
makes it easier to correlate presentation and implementation.

I removed bold font usage from everything except the hunk headers as I
really did not like the way bold font caused column alignments to become
out of whack within the diff viewer.  It also drew attention to the parts
of the file which were identically changed in both the index and in the
working directory, yet these are usually the parts I find myself caring
the least about.  So its very counter-intuitive.

Lines which are changed differently by both the index and the working
directory are now shown with background colors which span the entire line,
making these lines easier to pick out of the diff.  In general these are
the lines that appear to be more interesting to me when looking at the
3-way diff as they are the ones which contain recent and quite different
changes.

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