I tryed to answer this my self, but all I can find is too complicated or far away what I need.

Is it possible to use Git/Bitbucket/Mercurial/… to maintain a lot of websites on my local machine (NAS) and keep track of changes and upload the right files when editing is done?

I don't need an online repository at github or something like that, don't I?

Why I want this?

I'm a web developer and have to maintain about 50 websites. In the past I edited my files either on my local machine and uploaded it via FTP or directly on the remote server.

Both are unsafe.
In the first way I need to keep track what files are edited and upload all files. If I forget a file the page isn't up to date or worse the application/CMS wont work.

The second way isn't a good practice, because when something went wrong everybody on the web can see it ;-)