Working around a Vim bug that causes flickering.

If the user had a hidden buffer and a recent version of Vim, the screen would
flicker every time the user typed. This was caused by a Vim bug.

On every key press, we end up calling GetUnsavedAndCurrentBufferData(), which
calls GetBufferOption( buffer_object, 'ft' ). If the buffer_object represents a
hidden buffer, Vim would flicker.

This would happen because we'd call "buffer_object.options[ 'ft' ]" in recent
versions of Vim, and that line of code causes Vim to flicker. I don't know why.
We're extracting the 'ft' value without going through buffer_object.options, and
that works just fine.

Fixes #669.

python/ycm/vimsupport.py

@@ -51,9 +51,14 @@ def TextAfterCursor():
 # Note the difference between buffer OPTIONS and VARIABLES; the two are not
 # the same.
 def GetBufferOption( buffer_object, option ):
-  # The 'options' property is only available in recent (7.4+) Vim builds
-  if hasattr( buffer_object, 'options' ):
-    return buffer_object.options[ option ]
+  # NOTE: We used to check for the 'options' property on the buffer_object which
+  # is available in recent versions of Vim and would then use:
+  #
+  #   buffer_object.options[ option ]
+  #
+  # to read the value, BUT this caused annoying flickering when the
+  # buffer_object was a hidden buffer (with option = 'ft'). This was all due to
+  # a Vim bug. Until this is fixed, we won't use it.
 
   to_eval = 'getbufvar({0}, "&{1}")'.format( buffer_object.number, option )
   return GetVariableValue( to_eval )