Load plugins only once in plug#load (#616)

When loading 'deoplete.nvim' for the 2nd time during InsertEnter
manually, the `s:dobufread` (or `s:lod` itself) prevents it to work
properly - likely because the plugin gets resourced.

Maybe there could be a way to force this (and reload plugins always),
but by default it seems to make sense to skip already loaded plugins.

plug.vim

@@ -447,11 +447,15 @@ function! plug#load(...)
     let s = len(unknowns) > 1 ? 's' : ''
     return s:err(printf('Unknown plugin%s: %s', s, join(unknowns, ', ')))
   end
-  for name in a:000
+  let unloaded = filter(copy(a:000), '!get(s:loaded, v:val, 0)')
+  if !empty(unloaded)
+    for name in unloaded
       call s:lod([name], ['ftdetect', 'after/ftdetect', 'plugin', 'after/plugin'])
     endfor
-  call s:dobufread(a:000)
+    call s:dobufread(unloaded)
     return 1
+  end
+  return 0
 endfunction
 
 function! s:remove_triggers(name)

test/regressions.vader

@@ -1,5 +1,6 @@
 **********************************************************************
 Execute (#112 On-demand loading should not suppress messages from ftplugin):
+  call ResetPlug()
   call plug#begin('$PLUG_FIXTURES')
   Plug '$PLUG_FIXTURES/ftplugin-msg', { 'for': 'c' }
   call plug#end()
@@ -7,13 +8,14 @@ Execute (#112 On-demand loading should not suppress messages from ftplugin):
   redir => out
   tabnew a.c
   redir END
-  Assert stridx(out, 'ftplugin-c') >= 0
+  Assert stridx(out, 'ftplugin-c') >= 0, 'Unexpected output (1): '.out
 
 * The same applies to plug#load())
+  call ResetPlug()
   redir => out
   call plug#load('ftplugin-msg')
   redir END
-  Assert stridx(out, 'ftplugin-c') >= 0
+  Assert stridx(out, 'ftplugin-c') >= 0, 'Unexpected output (2): '.out
   q
 
 
@@ -89,10 +91,11 @@ Execute (#139-1 Using new remote branch):
   PlugUpdate
 
   unlet! g:foo g:bar g:baz
+  call ResetPlug()
   call plug#load('new-branch')
-  Assert exists('g:foo'),  'g:foo should be found'
+  Assert exists('g:foo'),  'g:foo should be found (1)'
-  Assert !exists('g:bar'), 'g:bar should not be found'
+  Assert !exists('g:bar'), 'g:bar should not be found (1)'
-  Assert !exists('g:baz'), 'g:baz should not be found'
+  Assert !exists('g:baz'), 'g:baz should not be found (1)'
 
   " Create a new branch on origin
   call system('cd /tmp/vim-plug-test/new-branch && git checkout -b new &&'
@@ -110,10 +113,11 @@ Execute (#139-1 Using new remote branch):
   Assert @" !~? 'error', 'Should be able to use new remote branch: ' . @"
 
   unlet! g:foo g:bar g:baz
+  call ResetPlug()
   call plug#load('new-branch')
-  Assert exists('g:foo'),  'g:foo should be found'
+  Assert exists('g:foo'),  'g:foo should be found (2)'
-  Assert exists('g:bar'),  'g:bar should be found'
+  Assert exists('g:bar'),  'g:bar should be found (2)'
-  Assert !exists('g:baz'), 'g:baz should not be found'
+  Assert !exists('g:baz'), 'g:baz should not be found (2)'
 
   call PlugStatusSorted()
 
@@ -140,6 +144,7 @@ Execute (#139-2 Using yet another new remote branch):
   Assert @" !~? 'error', 'Should be able to use new remote branch: ' . @"
 
   unlet! g:foo g:bar g:baz
+  call ResetPlug()
   call plug#load('new-branch')
   Assert exists('g:foo'),  'g:foo should be found'
   Assert !exists('g:bar'), 'g:bar should not be found'

test/workflow.vader

@@ -1128,6 +1128,7 @@ Execute (plug#helptags):
 **********************************************************************
 
 Execute (plug#load - invalid arguments):
+  call ResetPlug()
   AssertEqual 0, plug#load()
   AssertEqual 0, plug#load('non-existent-plugin')
   AssertEqual 0, plug#load('non-existent-plugin', 'another-non-existent-plugin')