merge: remove useless context from the end of a conflict.

merge: remove useless context from the end of a conflict.

If we are close to the end-of-file, then don't expect
too many unchanged lines.
And allow zero lines of context in the merge if  it is unchanged out
side the merge.

Update various tests to match - all clear improvements.

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