Trigger BufferVisit event only when buffer has changed

When selecting candidates during completion, Vim jumps to the preview
window (if enabled) and jumps back to the actual buffer, triggering
twice the BufEnter autocommand event. This results in YCM sending
two BufferVisit and one FileReadyToParse event notifications to
ycmd, which is completely unnecessary since the current buffer
did not change. We improve this by only sending these events when the
entered buffer allowed to be completed has changed.

autoload/youcompleteme.vim

@@ -28,6 +28,7 @@ let s:old_cursor_position = []
 let s:cursor_moved = 0
 let s:moved_vertically_in_insert_mode = 0
 let s:previous_num_chars_on_current_line = strlen( getline('.') )
+let s:previous_allowed_buffer_number = 0
 
 
 function! s:UsingPython2()
@@ -90,7 +91,8 @@ function! youcompleteme#Enable()
     " the user does :enew and then :set ft=something, we need to run buf init
     " code again.
     autocmd BufReadPre * call s:OnBufferReadPre( expand( '<afile>:p' ) )
-    autocmd BufRead,BufEnter,FileType * call s:OnBufferVisit()
+    autocmd BufRead,FileType * call s:OnBufferRead()
+    autocmd BufEnter * call s:OnBufferEnter()
     autocmd BufUnload * call s:OnBufferUnload( expand( '<afile>:p' ) )
     autocmd CursorHold,CursorHoldI * call s:OnCursorHold()
     autocmd InsertLeave * call s:OnInsertLeave()
@@ -118,7 +120,7 @@ function! youcompleteme#Enable()
   " triggering for the first loaded file. This should be the last commands
   " executed in this function!
   call s:OnBufferReadPre( expand( '<afile>:p' ) )
-  call s:OnBufferVisit()
+  call s:OnBufferRead()
 endfunction
 
 
@@ -324,7 +326,7 @@ function! s:TurnOffSyntasticForCFamily()
 endfunction
 
 
-function! s:AllowedToCompleteInCurrentFile()
+function! s:AllowedToCompleteInCurrentBuffer()
   if empty( &filetype ) ||
         \ getbufvar( winbufnr( winnr() ), "&buftype" ) ==# 'nofile' ||
         \ &filetype ==# 'qf'
@@ -343,6 +345,19 @@ function! s:AllowedToCompleteInCurrentFile()
 endfunction
 
 
+function! s:VisitedBufferRequiresReparse()
+  if !s:AllowedToCompleteInCurrentBuffer()
+    return 0
+  endif
+
+  if bufnr( '' ) ==# s:previous_allowed_buffer_number
+    return 0
+  endif
+  let s:previous_allowed_buffer_number = bufnr( '' )
+  return 1
+endfunction
+
+
 function! s:SetUpCommands()
   command! YcmRestartServer call s:RestartServer()
   command! YcmShowDetailedDiagnostic call s:ShowDetailedDiagnostic()
@@ -427,13 +442,13 @@ function! s:OnBufferReadPre(filename)
   endif
 endfunction
 
-function! s:OnBufferVisit()
+function! s:OnBufferRead()
   " We need to do this even when we are not allowed to complete in the current
-  " file because we might be allowed to complete in the future! The canonical
+  " buffer because we might be allowed to complete in the future! The canonical
   " example is creating a new buffer with :enew and then setting a filetype.
   call s:SetUpYcmChangedTick()
 
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -449,8 +464,18 @@ function! s:OnBufferVisit()
 endfunction
 
 
+function! s:OnBufferEnter()
+  if !s:VisitedBufferRequiresReparse()
+    return
+  endif
+
+  exec s:python_command "ycm_state.OnBufferVisit()"
+  call s:OnFileReadyToParse()
+endfunction
+
+
 function! s:OnBufferUnload( deleted_buffer_file )
-  if !s:AllowedToCompleteInCurrentFile() || empty( a:deleted_buffer_file )
+  if !s:AllowedToCompleteInCurrentBuffer() || empty( a:deleted_buffer_file )
     return
   endif
 
@@ -460,7 +485,7 @@ endfunction
 
 
 function! s:OnCursorHold()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -509,7 +534,7 @@ function! s:SetOmnicompleteFunc()
 endfunction
 
 function! s:OnCursorMovedInsertMode()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -546,7 +571,7 @@ endfunction
 
 
 function! s:OnCursorMovedNormalMode()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -556,7 +581,7 @@ endfunction
 
 
 function! s:OnInsertLeave()
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif
 
@@ -573,7 +598,7 @@ endfunction
 function! s:OnInsertEnter()
   let s:previous_num_chars_on_current_line = strlen( getline('.') )
 
-  if !s:AllowedToCompleteInCurrentFile()
+  if !s:AllowedToCompleteInCurrentBuffer()
     return
   endif