Revert "[window] pad lists with None so all are same size #2279"

This reverts commit bbfbb45.

tests/golden/addcol_window.tsv

@@ -1,5 +1,5 @@
 OrderDate	Region	Rep	Item	Units	Units_window	sum(Units_window)	Unit_Cost	Total
-2016-01-06	East	Jones	Pencil	95	[3] ; 95; 50	145	1.99	189.05
+2016-01-06	East	Jones	Pencil	95	[2] 95; 50	145	1.99	189.05
 2016-01-23	Central	Kivell	Binder	50	[3] 95; 50; 36	181	19.99	999.50
 2016-02-09	Central	Jardine	Pencil	36	[3] 50; 36; 27	113	4.99	179.64
 2016-02-26	Central	Gill	Pen	27	[3] 36; 27; 56	119	19.99	539.73
@@ -41,4 +41,4 @@ OrderDate	Region	Rep	Item	Units	Units_window	sum(Units_window)	Unit_Cost	Total
 2017-10-31	Central	Andrews	Pencil	14	[3] 57; 14; 11	82	1.29	18.06
 2017-11-17	Central	Jardine	Binder	11	[3] 14; 11; 94	119	4.99	54.89
 2017-12-04	Central	Jardine	Binder	94	[3] 11; 94; 28	133	19.99	1879.06
-2017-12-21	Central	Andrews	Binder	28	[3] 94; 28; 	122	4.99	139.72
+2017-12-21	Central	Andrews	Binder	28	[2] 94; 28	122	4.99	139.72

visidata/features/window.py

@@ -3,30 +3,13 @@
 
 from visidata import Sheet, Column, vd, asyncthread, Progress
 
-def getslice(L, i, j):
-    'Return L[i:j], except if i < 0 or j >= len(L), it pads with None, so the returned list always has j-i items.'
-    r = []
-    if i >= 0 and j < len(L):
-        return L[i:j]
-
-    a = i if i >= 0 else None
-    b = j if j < len(L) else None
-
-    for i in range(0, -i):
-        r.append(None)
-
-    r.extend(L[a:b])
-
-    for i in range(0, j-len(L)):
-        r.append(None)
-
-    return r
-
 @Sheet.api
 def window(sheet, before:int=0, after:int=0):
     '''Generate (row, list[row]) for each row in *sheet*, where list[row] is the rows within *before* number of rows before and *after* number of rows after the *row*.  The *row* itself is always included in the list.'''
     for i, r in enumerate(sheet.rows):
-        yield r, getslice(sheet.rows, i-before, i+after+1)
+        a = max(0, i-before) if before >= 0 else None
+        b = (i+after+1) if after >= 0 else None
+        yield r, sheet.rows[a:b]
 
 
 @Column.api