Avoid over-selecting when include_surrounding_whitespace

[yjyao]

Fixes #575
Fixes #850

The fix

When expanding to include surrounding whitespaces,

• Under the "charwise" selection_mode, only include whitespaces from the current line;
• Under the "linewise" selection_mode, include surrounding blank lines, but not non-empty lines with leading/trailing whitespaces.

Important

I'm not sure what's the intended usecase for blockwise selection when it comes to selecting textobjects, and how include_surrounding_whitespace is designed to work in that mode. This change currently DOES NOT handle blockwise mode correctly. PLEASE let me know what's the best way to handle it.

Tests

The following examples come from issues #575 and #850:

• Delete @function.outer deletes more than it should #575 (comment)
• FR: include_surrounding_whitespace should adjust whitespace characters included based on mode #850 (comment)

---

     1	def a():
     2	  x = 1
     3	
     4	  def b():
     5	
     6	    pass
     7	
     8	  pass

5Gvaf (af for linewise @function.outer)
selects lines [4,7],
correctly omitting line 8 which starts with leading whitespaces.

---

     1	def a():
     2	  pass␣␣
     3	
     4	def c():
     5	  pass

5Gvaf (af for linewise @function.outer)
selects lines [3,5],
correctly omitting line 2 which ends with trailing whitespaces.

---

     1	arr = [
     2	  x,  # comment␣␣
     3	  y,
     4	]

5Gvac (ac for charwise @comment.outer)
selects only # comment ,
correctly omitting the leading whitespaces on line 3.