t9001 (send-email): Do not use hardcoded /bin/sh in test

Scriptlets used form inside this test began with hardcoded "#!/bin/sh".
By setting SHELL_PATH the user is already telling us that what the vendor
has in /bin/sh isn't POSIX enough, and we really should try to honor that
request.

Originally noticed by SungHyun Nam who later tested this patch and
verified that it fixes the issue on Solaris 9.

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

t/t9001-send-email.sh

@@ -13,7 +13,7 @@ test_expect_success \
 
 test_expect_success \
     'Setup helper tool' \
-    '(echo "#!/bin/sh"
+    '(echo "#!$SHELL_PATH"
       echo shift
       echo output=1
       echo "while test -f commandline\$output; do output=\$((\$output+1)); done"
@@ -138,7 +138,7 @@ test_expect_success 'Valid In-Reply-To when prompting' '
 '
 
 test_expect_success 'setup fake editor' '
-	(echo "#!/bin/sh" &&
+	(echo "#!$SHELL_PATH" &&
 	 echo "echo fake edit >>\"\$1\""
 	) >fake-editor &&
 	chmod +x fake-editor
@@ -235,7 +235,7 @@ test_expect_success 'sendemail.cc unset' '
 
 test_expect_success '--compose adds MIME for utf8 body' '
 	clean_fake_sendmail &&
-	(echo "#!/bin/sh" &&
+	(echo "#!$SHELL_PATH" &&
 	 echo "echo utf8 body: àéìöú >>\"\$1\""
 	) >fake-editor-utf8 &&
 	chmod +x fake-editor-utf8 &&
@@ -254,7 +254,7 @@ test_expect_success '--compose adds MIME for utf8 body' '
 
 test_expect_success '--compose respects user mime type' '
 	clean_fake_sendmail &&
-	(echo "#!/bin/sh" &&
+	(echo "#!$SHELL_PATH" &&
 	 echo "(echo MIME-Version: 1.0"
 	 echo " echo Content-Type: text/plain\\; charset=iso-8859-1"
 	 echo " echo Content-Transfer-Encoding: 8bit"