userdiff: fix some corner cases in dts regex

While reviewing some dts diffs recently I noticed that the hunk header
logic was failing to find the containing node. This is because the regex
doesn't consider properties that may span multiple lines, i.e.

	property = <something>,
		   <something_else>;

and it got hung up on comments inside nodes that look like the root node
because they start with '/*'. Add tests for these cases and update the
regex to find them. Maybe detecting the root node is too complicated but
forcing it to be a backslash with any amount of whitespace up to an open
bracket seemed OK. I tried to detect that a comment is in-between the
two parts but I wasn't happy so I just dropped it.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Reviewed-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>