checkout: improve error messages for -b with extra argument

When we try to create a branch "foo" based on "origin/master" and give
git commit -b an extra unsupported argument "bar", it confusingly
reports:

   $ git checkout -b foo origin/master bar
   fatal: 'bar' is not a commit and a branch 'foo' cannot be created from it

   $ git checkout --track -b foo origin/master bar
   fatal: 'bar' is not a commit and a branch 'foo' cannot be created from it

That's wrong, because it very well understands that "origin/master" is
supposed to be the start point for the new branch and not "bar".  Check
if we got a commit and show more fitting messages in that case instead:

   $ git checkout -b foo origin/master bar
   fatal: Cannot update paths and switch to branch 'foo' at the same time.

   $ git checkout --track -b foo origin/master bar
   fatal: '--track' cannot be used with updating paths

Original-patch-by: Jeff King <peff@peff.net>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -1689,7 +1689,7 @@ static int checkout_main(int argc, const char **argv, const char *prefix,
 		 * Try to give more helpful suggestion.
 		 * new_branch && argc > 1 will be caught later.
 		 */
-		if (opts->new_branch && argc == 1)
+		if (opts->new_branch && argc == 1 && !new_branch_info.commit)
 			die(_("'%s' is not a commit and a branch '%s' cannot be created from it"),
 				argv[0], opts->new_branch);
 

t/t2018-checkout-branch.sh

@@ -265,7 +265,7 @@ test_expect_success 'checkout -b rejects an invalid start point' '
 	test_i18ngrep "is not a commit" err
 '
 
-test_expect_failure 'checkout -b rejects an extra path argument' '
+test_expect_success 'checkout -b rejects an extra path argument' '
 	test_must_fail git checkout -b branch5 branch1 file1 2>err &&
 	test_i18ngrep "Cannot update paths and switch to branch" err
 '

t/t2027-checkout-track.sh

@@ -16,7 +16,7 @@ test_expect_success 'checkout --track -b creates a new tracking branch' '
 	test $(git config --get branch.branch1.merge) = refs/heads/master
 '
 
-test_expect_failure 'checkout --track -b rejects an extra path argument' '
+test_expect_success 'checkout --track -b rejects an extra path argument' '
 	test_must_fail git checkout --track -b branch2 master one.t 2>err &&
 	test_i18ngrep "cannot be used with updating paths" err
 '