merge: remove useless context at the start of a conflict.

merge: remove useless context at the start of a conflict.

There are two fixes here.

The first allows the newline at the end of an unchanged region to be
the 'first' newline found so that if there are several others to
provide backup, it will be used.

The second allows the start-of-file to be strong enough context to
not require three full lines of explicit context.

Then we change a lot of the self-tests to match.  In each case the
result is a clear improvement.

Signed-off-by: NeilBrown <neilb@suse.de>