Do not use viminfo when running gentests
Add support for matchaddpos()

Fixes [#3]
Add support for patch 7.4.241

It is more efficient to use `submatch(, 1)` then trying to obtain the same
result using some weird hack.
Make it not possible to use this hack if submatch(, 1) is supported
Fix regression-difformat-withemptyfile test

Vim for some reason changed the way it calculates which line in one file
corresponds to which line in another file when using diff. So I have to unset
scrollbind to compensate that.