"git merge -Xtheirs" did not help content-level merge of binary
files; it should just take their version.  Also "*.jpg binary" in
the attributes did not imply they should use the binary ll-merge
driver.

* jc/ll-merge-binary-ours:
  ll-merge: warn about inability to merge binary files only when we can't
  attr: "binary" attribute should choose built-in "binary" merge driver
  merge: teach -Xours/-Xtheirs to binary ll-merge driver