t3400-rebase: Move detached HEAD check earlier
3ec7371f (Add two extra tests for git rebase, 2009-02-09) added a check
that rebase works on a detached HEAD. For this check, the last test
was picked, whose topic is to rebase a mode change. This rebase operation
amounts to a no-op on Windows/MinGW so that HEAD@{1} *is* modechange@{1},
and the intended detached HEAD test fails.
This patch moves the check earlier, into the group of tests that check
basic rebase properties.
Signed-off-by: Johannes Sixt <j6t@kdbg.org>