hg: add more extensions
Extract backup to a function
git, hg: symlink gitconfig and hgrc

Symlink configs instead of copying. Replace username/email update by
including a small local config extension.
Change email to the shorter one
vim, hg: in the end of hgcommit, add commit prefix

Source an extra script that should contain something like this:

    exe "normal ggiProjectPrefix: "

It will automatically insert a "ProjectPrefix: " string into an empty
commit message.
vim, hg: set textwidth unconditionally