Switch &shell during submodule update

Fix #481

plug.vim

@@ -722,15 +722,25 @@ function! s:assign_name()
   silent! execute 'f' fnameescape(name)
 endfunction
 
+function! s:chsh(swap)
+  let prev = [&shell, &shellredir]
+  if !s:is_win && a:swap
+    set shell=sh shellredir=>%s\ 2>&1
+  endif
+  return prev
+endfunction
+
 function! s:bang(cmd, ...)
   try
+    let [sh, shrd] = s:chsh(a:0)
     " FIXME: Escaping is incomplete. We could use shellescape with eval,
     "        but it won't work on Windows.
-    let cmd = a:0 > 0 ? s:with_cd(a:cmd, a:1) : a:cmd
+    let cmd = a:0 ? s:with_cd(a:cmd, a:1) : a:cmd
     let g:_plug_bang = '!'.escape(cmd, '#!%')
     execute "normal! :execute g:_plug_bang\<cr>\<cr>"
   finally
     unlet g:_plug_bang
+    let [&shell, &shellredir] = [sh, shrd]
   endtry
   return v:shell_error ? 'Exit status: ' . v:shell_error : ''
 endfunction
@@ -1823,10 +1833,7 @@ endfunction
 
 function! s:system(cmd, ...)
   try
-    let [sh, shrd] = [&shell, &shellredir]
+    let [sh, shrd] = s:chsh(1)
-    if !s:is_win
-      set shell=sh shellredir=>%s\ 2>&1
-    endif
     let cmd = a:0 > 0 ? s:with_cd(a:cmd, a:1) : a:cmd
     return system(s:is_win ? '('.cmd.')' : cmd)
   finally

test/workflow.vader

@@ -841,7 +841,12 @@ Execute (PlugUpdate!):
 Execute (When submodules are not initialized):
   call system(printf('cd %s && git submodule deinit subsubmodule', g:plugs['subsubmodule'].dir))
 
+^ #481 submodule update should use standard shell
+  let sh = &shell
+  set sh=/bin/echo
   silent PlugUpdate!
+  let &shell = sh
+  unlet sh
   q
   AssertEqual ' ', system(printf('cd %s && git submodule status', g:plugs['subsubmodule'].dir))[0],
     \ 'subsubmodule/subsubmodule should be initialized'