vcs-svn: Allow change nodes for root of tree (/)

vcs-svn: Allow change nodes for root of tree (/)

It is not uncommon for a svn repository to include change records for
properties at the top level of the tracked tree:

	Node-path:
	Node-kind: dir
	Node-action: change
	Prop-delta: true
	Prop-content-length: 43
	Content-length: 43

	K 10
	svn:ignore
	V 11
	build-area

	PROPS-END

Unfortunately a recent svn-fe change (vcs-svn: More dump format sanity
checks, 2010-11-19) causes such nodes to be rejected with the error
message

	fatal: invalid dump: path to be modified is missing

The repo_tree module does not keep a dirent for the root of the tree.
Add a block to the dump parser to take care of this case.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>