avoid STDERR diff3 messages when using wfMerge()

avoid STDERR diff3 messages when using wfMerge()

diff3 issues a warning to stderr if any of the files does not end with a
newline character.

TextContent::preSaveTransform() does normalize revision text by simply
calling trim().  Thus to avoid a diff3 error we simply apply the same
normalization and add a newline to please the command.

Change-Id: I7baa3df95dd70cbc865b2491ccc791e60f8b9e6e