[PATCH 0 of 2] hgeditor: make vim close the diff window automatically when :wq'ing

[Itamar Ravid]

Hi list - as per issue #2124, currently when using hgeditor with vim, the diff window isn't automatically closed when 
:wq'ing in the commit message window. These two patches abstract the functionality of calling an editor into an edit function,
and change the arguments vim is called with to make the diff window a help buffer (shamelessly stolen from the wiki).

I am aware that users are supposed to change hgeditor to fit their needs, but this default functionality makes hgeditor
more useful and is a feature most users would like.

Comment away! :-)