stdout and stderr cannot be swapped in Python 3 to use completion in
`input` on stderr. This commit rewrites code parts to use stdout
instead.

Before this commit, `config-update` was found by an shell alias. Now, it
its directory is added to the `PATH` variable.

Unstaged changes will be stashed and applied before the working copy
changes are applied to the config directory.

When switching branches internally, staged changes were lost (because of
git stash usage) -> Added `--index` to `git stash pop` to prevent this.

After closing the editor for the first config file the user can choose
another file.

It is better to use the user's preferred configuration via `~/.inputrc`.

New files were added to the currently checked out branch instead.

If both config directory and config symlink are located in `${HOME}`,
a relative link targets are used instead of absolute ones. This prevents
problems with changing home directory paths (e.g. if the home directory
is mounted via NFS).

Diff stats at the end of the updating process often used a wrong
`HEAD@{1}` reference. Now, before the update process the old HEAD is
saved explicitly.

Branches were synced with remote ones but not the working directory;
that is fixed now.
Improved handling of unpushed changes (for example when no network
connection was available when updating files): Branches are merged and
not reset any more.
Uncomitted changes are stashed before applying any other changes and
restored afterwards.

Config files in `${HOME}` shall be stored relative to `${HOME}` to
permit different home directory pathes on different machines.