Fixed spurious window bug. see commit details below.

Calling bd! closes the window and clears the 'diff' option for the
remaining window, which therefore does not get deleted.
Here I am not testing for the 'diff' option and I close the window
indiscriminately, but there's probably a better way.
Conflict with fugitive, it clears the 'diff' options
Merge remote-tracking branch 'grota/master'