🐎 Don't get every line to check for soft-tabs

Getting all the lines requires SharedString to construct a big array.
It's more efficient to check them one line at a time.

src/text-buffer.coffee

@@ -593,8 +593,8 @@ class TextBuffer
   #
   # Returns a {Boolean},
   usesSoftTabs: ->
-    for line in @getLines()
-      if match = line.match(/^\s/)
+    for row in [0..@getLastRow()]
+      if match = @lineForRow(row).match(/^\s/)
         return match[0][0] != '\t'
     undefined