t0000: do not get self-test disrupted by environment warnings

The test framework test-lib.sh itself would want to give warnings
and hints, e.g. when it sees a deprecated environment variable is in
use that we want to encourage users to migrate to another variable.

The self-test of test framework done in t0000 however do not expect
to see these warnings and hints, so depending on the settings of
environment variables, a running test may or may not produce these
messages to the standard error output, breaking the expectations of
self-test test framework does on itself.  Here is what we see:

    $ TEST_GIT_INDEX_VERSION=4 sh t0000-basic.sh -i -v
    ...
    'err' is not empty, it contains:
    warning: TEST_GIT_INDEX_VERSION is now GIT_TEST_INDEX_VERSION
    hint: set GIT_TEST_INDEX_VERSION too during the transition period
    not ok 5 - pretend we have a fully passing test suite

The following quick attempt to work it around does not work, because
some tests in t0000 do want to see expected errors from the test
framework itself.

         t/t0000-basic.sh | 2 +-
         1 file changed, 1 insertion(+), 1 deletion(-)

        diff --git a/t/t0000-basic.sh b/t/t0000-basic.sh
        index 850f651e4e..88c6ed4696 100755
        --- a/t/t0000-basic.sh
        +++ b/t/t0000-basic.sh
        @@ -88,7 +88,7 @@ _run_sub_test_lib_test_common () {
                        '

                        # Point to the t/test-lib.sh, which isn't in ../ as usual
        -		. "\$TEST_DIRECTORY"/test-lib.sh
        +		. "\$TEST_DIRECTORY"/test-lib.sh >/dev/null 2>&1
                        EOF
                        cat >>"$name.sh" &&
                        chmod +x "$name.sh" &&

There are a few possible ways to work this around:

 * We could strip the warning: and hint: unconditionally from the
   error output before the error messages are checked in the
   self-test (helper functions check_sub_test_lib_test_err and
   check_sub_test_lib_test); the problem with this approach is that
   it will make it impossible to write self-tests to ensure that
   right warnings and hints are given.

 * We could force a sane environment settings before the test helper
   _run_sub_test_lib_test_common dot-sources test-lib.sh; the
   problem with this approach is that _run_sub_test_lib_test_common
   now needs to be aware of what pairs of environment variables are
   checked in test-lib.sh using check_var_migration helper.

The final patch I came up with is probably the solution that is
least bad.  Set a variable to tell test-lib.sh that we are running
a self-test, so that various pieces in test-lib.sh can react to keep
the output stable.

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

t/t0000-basic.sh

@@ -87,6 +87,10 @@ _run_sub_test_lib_test_common () {
 		passing metrics
 		'
 
+		# Tell the framework that we are self-testing to make sure
+		# it yields a stable result.
+		GIT_TEST_FRAMEWORK_SELFTEST=t &&
+
 		# Point to the t/test-lib.sh, which isn't in ../ as usual
 		. "\$TEST_DIRECTORY"/test-lib.sh
 		EOF

t/test-lib.sh

@@ -135,9 +135,17 @@ GIT_TRACE_BARE=1
 export GIT_TRACE_BARE
 
 check_var_migration () {
+	# the warnings and hints given from this helper depends
+	# on end-user settings, which will disrupt the self-test
+	# done on the test framework itself.
+	case "$GIT_TEST_FRAMEWORK_SELFTEST" in
+	t)	return ;;
+	esac
+
 	old_name=$1 new_name=$2
 	eval "old_isset=\${${old_name}:+isset}"
 	eval "new_isset=\${${new_name}:+isset}"
+
 	case "$old_isset,$new_isset" in
 	isset,)
 		echo >&2 "warning: $old_name is now $new_name"