gitk: Add a tree-browsing mode

You can now select whether you want to see the patch for a commit
or the whole tree.  If you select the tree, gitk will now display
the commit message plus the contents of one file in the bottom-left
pane, when you click on the name of the file in the bottom-right pane.

Signed-off-by: Paul Mackerras <paulus@samba.org>