t7406: fix misleading "rev-parse --max-count=1 HEAD"

The test happened to use "rev-parse --max-count=1 HEAD" consistently
to prepare the expected output and the actual output, so the
comparison between them gave us a correct success/failure because
both output had irrelevant "--max-count=1" in it.

But that is not an excuse to keep it broken.  Replace it a more
meaningful construct "rev-parse --verify HEAD".

Noticed by Daniel Graña while working on his submodule tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7406-submodule-update.sh

@@ -305,7 +305,7 @@ test_expect_success 'submodule update continues after checkout error' '
 	 git submodule init &&
 	 git commit -am "new_submodule" &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../expect
+	  git rev-parse --verify HEAD >../expect
 	 ) &&
 	 (cd submodule &&
 	  test_commit "update_submodule" file
@@ -322,7 +322,7 @@ test_expect_success 'submodule update continues after checkout error' '
 	 git checkout HEAD^ &&
 	 test_must_fail git submodule update &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../actual
+	  git rev-parse --verify HEAD >../actual
 	 ) &&
 	 test_cmp expect actual
 	)
@@ -351,7 +351,7 @@ test_expect_success 'submodule update continues after recursive checkout error'
 	  test_commit "update_submodule_again_again" file
 	 ) &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../expect &&
+	  git rev-parse --verify HEAD >../expect &&
 	  test_commit "update_submodule2_again" file
 	 ) &&
 	 git add submodule &&
@@ -366,7 +366,7 @@ test_expect_success 'submodule update continues after recursive checkout error'
 	 ) &&
 	 test_must_fail git submodule update --recursive &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../actual
+	  git rev-parse --verify HEAD >../actual
 	 ) &&
 	 test_cmp expect actual
 	)
@@ -398,12 +398,12 @@ test_expect_success 'submodule update exit immediately in case of merge conflict
 	 ) &&
 	 git checkout HEAD^ &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../expect
+	  git rev-parse --verify HEAD >../expect
 	 ) &&
 	 git config submodule.submodule.update merge &&
 	 test_must_fail git submodule update &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../actual
+	  git rev-parse --verify HEAD >../actual
 	 ) &&
 	 test_cmp expect actual
 	)
@@ -432,12 +432,12 @@ test_expect_success 'submodule update exit immediately after recursive rebase er
 	 ) &&
 	 git checkout HEAD^ &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../expect
+	  git rev-parse --verify HEAD >../expect
 	 ) &&
 	 git config submodule.submodule.update rebase &&
 	 test_must_fail git submodule update &&
 	 (cd submodule2 &&
-	  git rev-parse --max-count=1 HEAD > ../actual
+	  git rev-parse --verify HEAD >../actual
 	 ) &&
 	 test_cmp expect actual
 	)