calcHash: use correct line endings

Make calcHash() respect the buffer's file endings (unix vs dos), to make
its calculation of the file size consistent with how we calculate it in
other cases (i.e. when opening or saving the file) and with the
`fastdirty` option documentation, i.e. make calcHash() return
ErrFileTooLarge if and only if the exact file size exceeds 50KB.

internal/buffer/buffer.go

@@ -666,7 +666,11 @@ func calcHash(b *Buffer, out *[md5.Size]byte) error {
 		size += n
 
 		for _, l := range b.lines[1:] {
-			n, _ = h.Write([]byte{'\n'})
+			if b.Endings == FFDos {
+				n, _ = h.Write([]byte{'\r', '\n'})
+			} else {
+				n, _ = h.Write([]byte{'\n'})
+			}
 			size += n
 			n, _ = h.Write(l.data)
 			size += n