ll-merge: warn about inability to merge binary files only when we can't

ll-merge: warn about inability to merge binary files only when we can't

When a path being merged is auto detected to be a binary file, we
warned "Cannot merge binary files" before switching to activate the
binary ll-merge driver.  When we are merging with the -Xours/theirs
option, however, we know what the "clean" merge result is, and the
warning is inappropriate.

In addition, when the path is explicitly marked as a binary file,
this warning was not issued, even though without -Xours/theirs, we
cannot cleanly automerge such a path, which was inconsistent.

Move the warning code from ll_xdl_merge() to ll_binary_merge(), and
issue the message only when we cannot cleanly automerge.

Signed-off-by: Junio C Hamano <gitster@pobox.com>