Default to shallow clone/fall back to regular, don't plug vim-plug:

vim-plug repo is pretty big (8.8M at the time of this commit),
so it makes sense to only fetch the HEAD of master (or cURL
using raw.githubusercontent.com if available). If however shallow
clone is not supported due to proxying/mirroring, regular git clone
will be used.

Maintaining vim-plug as a plugin can be a bad idea for several reasons,
please see the vim-plug FAQ:

https://github.com/junegunn/vim-plug/wiki/faq#shouldnt-vim-plug-update-itself-on-plugupdate-like-vundle

Author: timss

.vimrc

@@ -15,7 +15,9 @@
     " Automatic installation
     " https://github.com/junegunn/vim-plug/wiki/faq#automatic-installation
     if empty(glob('~/.vim/autoload/plug.vim'))
-        silent !git clone https://github.com/junegunn/vim-plug.git $HOME/.vim/bundle/vim-plug
+        let clone_details = "https://github.com/junegunn/vim-plug.git $HOME/.vim/bundle/vim-plug"
+        silent call system("git clone --depth 1 ". clone_details)
+        if v:shell_error | silent call system("git clone " . clone_details) | endif
         silent !ln -s $HOME/.vim/bundle/vim-plug/plug.vim $HOME/.vim/autoload/plug.vim
         autocmd VimEnter * PlugInstall --sync | source $MYVIMRC
     endif
@@ -42,9 +44,6 @@
     " Default to same plugin directory as vundle etc
     call plug#begin('~/.vim/bundle')
 
-    " Maintain vim-plug as a plugin as well
-    Plug 'junegunn/vim-plug'
-
     " <Tab> everything!
     Plug 'ervandew/supertab'