git checkout: do not allow switching to a tree-ish that is not a commit

"git checkout -b newbranch $commit^{tree}" mistakenly created a new branch
rooted at the current HEAD, because in that case, the two structure fields
used to see if the command was invoked without any argument (hence it
needs to default to checking out the HEAD) were populated incorrectly.

Upon seeing a command line argument that we took as a rev, we should store
that string in new.name, even if that does not name a commit.  This will
correctly trigger the existing safety logic.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: Daniel Barkalow <barkalow@iabervon.org>

builtin-checkout.c

@@ -515,8 +515,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		argv++;
 		argc--;
 
+		new.name = arg;
 		if ((new.commit = lookup_commit_reference_gently(rev, 1))) {
-			new.name = arg;
 			setup_branch_path(&new);
 			if (resolve_ref(new.path, rev, 1, NULL))
 				new.commit = lookup_commit_reference(rev);

t/t2011-checkout-invalid-head.sh

@@ -10,6 +10,10 @@ test_expect_success 'setup' '
 	git commit -m initial
 '
 
+test_expect_success 'checkout should not start branch from a tree' '
+	test_must_fail git checkout -b newbranch master^{tree}
+'
+
 test_expect_success 'checkout master from invalid HEAD' '
 	echo 0000000000000000000000000000000000000000 >.git/HEAD &&
 	git checkout master --