Fix and improve move detection.
Move detection had some issues that were a result of a stale variable

being accessed. When setting metadata, a previous variable was used,

instead of the instance we were currently operating on. This led to a

bunch of move information getting tacked onto the wrong line and erasing

things. That's been fixed, so move information can be better trusted.
Move detection was also picking up lots of little lines we really didn't

care about. It now has a threshold. It must be at least 2 lines of code,

or 1 line at least 20 characters in length. We may decide to tweak these

later on.
There was also an off-by-one with move ranges. If a range ended with a

line that wasn't considered interesting enough (< 4 characters), but the

range otherwise passed validation, that uninteresting line would be

included. This isn't always bad (might catch an ending brace, for

example), but it's not intended, and shouldn't be provided unless we

design for it.
I also made the move flags more consistent between the left-hand-side

and right-hand-side of the diff viewer. Both the "moved from" and "moved

to" flags are now anchored on the left of their side, so it's easier to

see what lines they span. Basically, I'm applying the new changes

recently made for the right-hand side to the left.

Unit tests pass.
Tested with the diff on /r/4919, which was full of junk and broken

move information before. It looks reasonable now.