Makesymlink does not break previous calls

If makesymlink is called, it no longer breaks previous calls, i.e.,
files that already have been copied to the dotfiles_old-folder are not
copied again (which would overwrite the backup with the symlink!). This
allows adding files to the repo, at which time a call to makesymlinks
would only add the new stuff.

Two things should be mentioned:
- it would be nice if this would be called automatically when pulling
(or at least when this file changes)
- if the dotfile is replaced in the home directory, it will not be under
revision any longer (as the script still detects the file in the
old-folder), so be careful with that
Add gitconfig-stuff to the repository

Mainly the aliases (and hist-alias) are handy in git. However, if other
nice tricks are discovered, these can now be added to be used on any
machine.

Note, the config-file itself has not been put under revision control, as
some machine-specific stuff can still be there (and who knows how git
automatically writes in this file). Especially the user and email can be
different on every machine. So, we opted for the use of "include" which
has to be manually added to the config-file, pointing at our
extension-file which *is* under revision control.