Move test for g:load_black to improve plugin performance (GH-2896)

If a vim/neovim user wishes to suppress loading the vim plugin by
setting g:load_black in their VIMRC (for me, Arch linux automatically
adds the plugin to Neovim's RTP, even though I'm not using it), the
current location of the test comes after a call to has('python3'). This
adds, in my tests, between 35 and 45 ms to Vim load time (which I know
isn't a lot but it's also unnecessary). Moving the call to
`exists('g:load_black')` to before the call to `has('python3')` removes
this unnecessary test and speeds up loading.

Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>

CHANGES.md

@@ -36,6 +36,8 @@
 
 <!-- For example, Docker, GitHub Actions, pre-commit, editors -->
 
+- Move test to disable plugin in Vim/Neovim, which speeds up loading (#2896)
+
 ### Output
 
 <!-- Changes to Black's terminal output and error messages -->

plugin/black.vim

@@ -15,6 +15,10 @@
 "  1.2:
 "    - use autoload script
 
+if exists("g:load_black")
+  finish
+endif
+
 if v:version < 700 || !has('python3')
     func! __BLACK_MISSING()
         echo "The black.vim plugin requires vim7.0+ with Python 3.6 support."
@@ -25,10 +29,6 @@ if v:version < 700 || !has('python3')
     finish
 endif
 
-if exists("g:load_black")
-  finish
-endif
-
 let g:load_black = "py1.0"
 if !exists("g:black_virtualenv")
   if has("nvim")