Please disallow cloning into standard runtime folders

[chrisbra]

Hi,
I have received now for the second time a bug report about my plugin misbehaving. Turned out, this was due to the fact that the user cloned into ~/.vim/plugin folder. That is certainly the wrong thing to do and will most likely make most plugins fail miserably.

So please disallow cloning at least into the plugin folder.

References:

• corresponding plug issue and fix
• Vim-Airline Issue 1440
• Vim-Airline Issue 1712
• Vim doc-patch

Thanks.