git-gui: Implemented file browser and incremental blame.

git-gui: Implemented file browser and incremental blame.

This rather huge change provides a browser for the current branch.  The
browser simply shows the contents of tree HEAD, and lets the user drill
down through the tree.  The icons used really stink, as I just copied in
icon which we already had.  I really need to replace the file_dir and
file_uplevel icons with something more useful.

If the user double clicks on a file within the browser we open it in
a blame viewer.  This makes use of the new incremental blame feature
that Linus just added yesterday to core Git.  Fortunately the feature
will be in 1.5.0 final so we can rely on having it available here.

Since the blame engine is incremental the user will get blame data
for groups which can be determined early.  Git will slowly fill in
the remaining lines as it goes.

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