rebase: give a better error message for bogus branch

When you give a non-existent branch to git-rebase, it spits out the usage. This can be confusing, since you may understand the usage just fine, but simply have made a mistake in the branch name. Before: $ git rebase origin bogus Usage: git rebase ... After: $ git rebase origin bogus fatal: no such branch: bogus Usage: git rebase ... Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>