Prefer del/ins over renames

There was some weird bug that caused the diff between

a + b

and

a + b
c + d

to generate a bad backtrace. Switching the order
seemed to fix it.
Add "text-mode" hack

It's a hack to re-use "gen-html" and thus
not have to remake the hover effect. A better
text diff tool would use a custom generator
without the javascript syntax stuff.

Pretty cool how well it works though.
Add `git difftool` compatible script/server

Kinda cool. With some work, could probably
just do the diff in v8, but not having the
HTML layout engine at my disposal for rendering
would be painful. Maybe just for "text mode" diffs?
Click to synchronize scroll

TODO remove code duplication.
Add sync to espr.ls