tools: Add git diff.

- This is worth it afterall.
- Eris will get out of sync when the diffs are commited,
  but this is also true of git-blame and git-log.
- The user can refresh the old results with shift-r.

make-readme.py

@@ -7,7 +7,7 @@ import eris.tools as tools
 
 def main():
     all_tools = ([(["*"], tools.generic_tools() +
-                   [tools.git_blame, tools.git_log])] +
+                   [tools.git_diff, tools.git_blame, tools.git_log])] +
                  tools.TOOLS_FOR_EXTENSIONS)
     tool_set = set()
     extension_set = set()