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>

t/t7102-reset.sh

@@ -497,4 +497,12 @@ test_expect_success 'disambiguation (4)' '
 	test ! -f secondfile
 '
 
+test_expect_success 'reset with paths accepts tree' '
+	# for simpler tests, drop last commit containing added files
+	git reset --hard HEAD^ &&
+	git reset HEAD^^{tree} -- . &&
+	git diff --cached HEAD^ --exit-code &&
+	git diff HEAD --exit-code
+'
+
 test_done