parser: allow words starting with "diff" at beginning of line

The patch parser splits emails incorrectly when the comment contains a
line whose first word starts with "diff" (like "difficult" or
"different").

For a real-life example of an email that triggers this bug, see
http://patchwork.ozlabs.org/patch/135291/.

Fix the issue by checking for a space after "diff".  Add
DiffWordInCommentTest to the test suite.

Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>