First pass of diff index to workdir implementation

This is an initial version of git_diff_workdir_to_index.  It
also includes renaming some structures and some refactoring
of the existing code so that it could be shared better with
the new function.

This is not complete since it needs a rebase to get some
new odb functions from the upstream branch.

src/diff.h

@@ -18,7 +18,7 @@ struct git_diff_list {
 
 	/* the following are just used while processing the diff list */
 	git_buf          pfx;
-	git_status_t     mode;
+	git_status_t     status;
 };
 
 #endif