Auto merge of #2644 - micbou:improve-reparse-requirement, r=Valloric

[READY] Improve reparse requirement on BufEnter event

When opening a file, Vim triggers the `BufRead` event then the `BufEnter` one. By setting the `s:previous_allowed_buffer_number` variable in `s:AllowedToCompleteInBuffer`, we avoid a reparse on `BufEnter` after parsing the buffer on `BufRead`. This reduces the number of `BufferVisit` and `FileReadyToParse` requests sent to the server by one when opening a file.

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autoload/youcompleteme.vim

@@ -345,7 +345,11 @@ function! s:AllowedToCompleteInBuffer( buffer )
         \ has_key( g:ycm_filetype_whitelist, buffer_filetype )
   let blacklist_allows = !has_key( g:ycm_filetype_blacklist, buffer_filetype )
 
-  return whitelist_allows && blacklist_allows
+  let allowed = whitelist_allows && blacklist_allows
+  if allowed
+    let s:previous_allowed_buffer_number = bufnr( a:buffer )
+  endif
+  return allowed
 endfunction
 
 
@@ -355,15 +359,11 @@ endfunction
 
 
 function! s:VisitedBufferRequiresReparse()
-  if !s:AllowedToCompleteInCurrentBuffer()
+  if bufnr( '%' ) ==# s:previous_allowed_buffer_number
     return 0
   endif
 
-  if bufnr( '' ) ==# s:previous_allowed_buffer_number
+  return s:AllowedToCompleteInCurrentBuffer()
-    return 0
-  endif
-  let s:previous_allowed_buffer_number = bufnr( '' )
-  return 1
 endfunction