read-tree: "read-tree -m --empty" does not make sense
fb1bb965 ("read-tree: deprecate syntax without tree-ish args",
2010-09-10) wanted to deprecate "git read-tree" without any tree,
which used to be the way to empty the index, and encourage use of
"git read-tree --empty" instead.
However, when used with "-m", "--empty" does not make any sense,
either, simply because merging 0 trees will result in a different
error anyway.
Omit the deprecation warning and let the code to emit real error
message diagnose the error.
Signed-off-by: Junio C Hamano <gitster@pobox.com>