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>

t/t3400-rebase.sh

@@ -48,6 +48,10 @@ test_expect_success \
     'the rebase operation should not have destroyed author information' \
     '! (git log | grep "Author:" | grep "<>")'
 
+test_expect_success 'HEAD was detached during rebase' '
+     test $(git rev-parse HEAD@{1}) != $(git rev-parse my-topic-branch@{1})
+'
+
 test_expect_success 'rebase after merge master' '
      git reset --hard topic &&
      git merge master &&
@@ -85,10 +89,6 @@ test_expect_success 'rebase a single mode change' '
      GIT_TRACE=1 git rebase master
 '
 
-test_expect_success 'HEAD was detached during rebase' '
-     test $(git rev-parse HEAD@{1}) != $(git rev-parse modechange@{1})
-'
-
 test_expect_success 'Show verbose error when HEAD could not be detached' '
      : > B &&
      test_must_fail git rebase topic 2> output.err > output.out &&