Teach git diff-tree --stdin to diff trees

Teach git diff-tree --stdin to diff trees

When feeding trees on the command line, you can give exactly two
trees, not three nor one; --stdin now supports this "two tree" form on
its input, in addition to accepting lines with one or more commits.

When diffing trees (either specified on the command line or from the
standard input), the -s, -v, --pretty, --abbrev-commit, --encoding,
--no-commit-id, and --always options are ignored, since they do not
apply to trees; and the -m, -c, and --cc options are ignored since
they would be meaningful only with three or more trees, which is not
supported (yet).

Signed-off-by: Karl Hasselström <kha@treskal.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>