bestmatch: improve poor matches.

bestmatch: improve poor matches.

If the bestmatch doens't include all of the lines in a hunk, try to
find matches between the unmatch parts of the hunk, and surrounding
text in the original.

This is particularly useful when some of the context is just blank
lines.  The best-match algorithm might ignore blank lines because they
aren't big enough to justify much distance from the rest of the match.

This makes tests/linux/raid1-A/merge a lot nicer.

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