Ensure BufWritePost,FileChangedShellPost always process buffer.

Before this change the TabEnter/BufEnter logic was intertwined with
the logic for BufWritePost,FileChangedShellPost.

Separating the concerns fixes a minor bug where a TabEnter-BufEnter
combination led to the plugin not updating the diff the first time the
buffer was written.

plugin/gitgutter.vim

@@ -196,18 +196,35 @@ augroup gitgutter
   endif
 
   if g:gitgutter_eager
-    autocmd BufEnter,BufWritePost,FileChangedShellPost *
+    autocmd BufWritePost,FileChangedShellPost * call gitgutter#process_buffer(bufnr(''), 0)
+
+    " When you enter a new tab, BufEnter is only fired if the buffer you enter
+    " is not the one you came from.
+    "
+    " For example:
+    "
+    "   `:tab split` fires TabEnter but not BufEnter.
+    "   `:tab new`   fires TabEnter and BufEnter.
+    "
+    " As and when both TabEnter and BufEnter are fired, we do not want to
+    " process the entered buffer twice.  We avoid this by setting and clearing
+    " a flag.
+
+    autocmd BufEnter *
           \  if gettabvar(tabpagenr(), 'gitgutter_didtabenter') |
           \   call settabvar(tabpagenr(), 'gitgutter_didtabenter', 0) |
           \ else |
           \   call gitgutter#process_buffer(bufnr(''), 0) |
           \ endif
+
     autocmd TabEnter *
           \ call settabvar(tabpagenr(), 'gitgutter_didtabenter', 1) |
           \ call gitgutter#all()
+
     if !has('gui_win32')
       autocmd FocusGained * call gitgutter#all()
     endif
+
   else
     autocmd BufRead,BufWritePost,FileChangedShellPost * call gitgutter#process_buffer(bufnr(''), 0)
   endif