[MERGE][0.15] Handle empty merge directive texts

[John Arbash Meinel]

John Arbash Meinel has voted +1 (conditional).
Status is now: Conditionally approved
Comment:
The patch is fine. Though I wonder if we want to pass the whole set of 
lines, rather than just the first one.

Considering you now will search through the entire file for a merge 
directive header, it seems certainly possible that this would not help a 
user find the problem.

It is minor, though. As this fits most use cases.

For details, see: 
http://bundlebuggy.aaronbentley.com/request/%3C46034D69.3010702%40utoronto.ca%3E