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>