[MERGE][0.15] Handle empty merge directive texts

[Aaron Bentley]

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> The patch is fine.

Please don't use +1 conditional when you think the patch is fine.
Conditional means that there is something I must change in order to have
your +1.

> 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.

I certainly don't want to see a mess-o-lines on the commandline when a
merge directive fails.  I think in the vast majority of cases, spewing
entire patches at users will be unhelpfully overwhelming.  It's like
drinking from a firehose.

The first line gives a pretty clear idea of what they just tried to
merge.  If they want to look at the whole thing, they would probably
prefer to do so in a pager or editor.

Aaron
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