t3400-rebase: Move detached HEAD check earlier

t3400-rebase: Move detached HEAD check earlier

Short story: There is a section in t3400 that tests fundamental rebase
properties.  3ec7371f (Add two extra tests for git rebase, 2009-02-09)
added a check that rebase works on a detached HEAD, but the test was put
near the end of the file.  This moves it to a more suitable place.

Long story: The test that preceded the one in question tests that a
rebased commit degrades from a content change with mode change to a
mere mode change.  But on Windows, where we have core.filemode=false,
the original commit did not record the mode change, and so the rebase
operation did not rebase anything.  This caused the subsequent detached
HEAD test to fail.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>