Update async.vim

[k-takata]

Import the latest version of async.vim. This should fix #131.

[k-takata]

It seems to hang up on Neovim. Why?

[matveyt]

It seems to hang up on Neovim. Why?

It looks like the problem is due to mixing slashes and backslashes on Windows (see for example, https://github.com/neovim/neovim/issues/13787).

With the following patch everything looks all right:

diff --git a/autoload/minpac.vim b/autoload/minpac.vim
index e28716c..d11f148 100644
--- a/autoload/minpac.vim
+++ b/autoload/minpac.vim
@@ -37,6 +37,9 @@ function! minpac#init(...) abort
   if l:packdir ==# ''
     " If 'dir' is not specified, the first directory of 'packpath' is used.
     let l:packdir = split(&packpath, ',')[0]
+    if has('nvim') && exists('+shellslash') && &shellslash
+      let l:packdir = tr(l:packdir, '\', '/')
+    endif
   endif
 
   let l:opt.minpac_dir = l:packdir . '/pack/' . l:opt.package_name