Ensure start row is always >= 0

Previously if a file was a single line
it couldn't be deleted using the delete
line command because the start row would
index one row back which would be -1 and
therefore not be able to report a length
for use in the delete range.

spec/app/edit-session-spec.coffee

@@ -1508,3 +1508,21 @@ describe "EditSession", ->
       expect(buffer.getLineCount()).toBe(13)
       editSession.deleteLine()
       expect(buffer.getLineCount()).toBe(4)
+
+    it "deletes the entire file from the bottom up", ->
+      count = buffer.getLineCount()
+      expect(count).toBeGreaterThan(0)
+      for line in [0...count]
+        editSession.getLastCursor().moveToBottom()
+        editSession.deleteLine()
+      expect(buffer.getLineCount()).toBe(1)
+      expect(buffer.getText()).toBe('')
+
+    it "deletes the entire file from the top down", ->
+      count = buffer.getLineCount()
+      expect(count).toBeGreaterThan(0)
+      for line in [0...count]
+        editSession.getLastCursor().moveToTop()
+        editSession.deleteLine()
+      expect(buffer.getLineCount()).toBe(1)
+      expect(buffer.getText()).toBe('')

src/app/buffer.coffee

@@ -168,7 +168,10 @@ class Buffer
     startPoint = null
     endPoint = null
     if end == @getLastRow()
-      startPoint = [start - 1, @lineLengthForRow(start - 1)]
+      if start > 0
+        startPoint = [start - 1, @lineLengthForRow(start - 1)]
+      else
+        startPoint = [start, 0]
       endPoint = [end, @lineLengthForRow(end)]
     else
       startPoint = [start, 0]