reset $sha1 $pathspec: require $sha1 only to be treeish

reset $sha1 $pathspec: require $sha1 only to be treeish

Resetting with paths does not update HEAD and there is nothing else
that a commit should be needed for. Relax the argument parsing so only
a tree is required.

The sha1 is only passed to read_from_tree(), which already only
requires a tree.

The "rev" variable we pass to run_add_interactive() will resolve to a
tree. This is fine since interactive_reset only needs the parameter to
be a treeish and doesn't use it for display purposes.

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>