tests: do not let lazy prereqs inside test_expect_* turn off tracing

The `test_expect_*` functions use `test_eval_` and so does
`test_run_lazy_prereq_`. If tracing is enabled via the `-x` option,
`test_eval_` turns on tracing while evaluating the code block, and turns
it off directly after it.

This is unwanted for nested invocations.

One somewhat surprising example of this is when running a test that
calls `test_i18ngrep`: that function requires the `C_LOCALE_OUTPUT`
prereq, and that prereq is a lazy one, so it is evaluated via
`test_eval_`, the command tracing is turned off, and the test case
continues to run _without tracing the commands_.

Another somewhat surprising example is when one lazy prereq depends on
another lazy prereq: the former will call `test_have_prereq` with the
latter one, which in turn calls `test_eval_` and -- you guessed it --
tracing (if enabled) will be turned off _before_ returning to evaluating
the other lazy prereq.

As we will introduce just such a scenario with the GPG, GPGSM and
RFC1991 prereqs, let's fix that by introducing a variable that keeps
track of the current trace level: nested `test_eval_` calls will
increment and then decrement the level, and only when it reaches 0, the
tracing will _actually_ be turned off.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t0000-basic.sh

@@ -833,6 +833,19 @@ then
 	exit 1
 fi
 
+test_expect_success 'lazy prereqs do not turn off tracing' "
+	run_sub_test_lib_test lazy-prereq-and-tracing \
+		'lazy prereqs and -x' -v -x <<-\\EOF &&
+	test_lazy_prereq LAZY true
+
+	test_expect_success lazy 'test_have_prereq LAZY && echo trace'
+
+	test_done
+	EOF
+
+	grep 'echo trace' lazy-prereq-and-tracing/err
+"
+
 test_expect_success 'tests clean up even on failures' "
 	run_sub_test_lib_test_err \
 		failing-cleanup 'Failing tests with cleanup commands' <<-\\EOF &&

t/test-lib.sh

@@ -882,6 +882,7 @@ maybe_setup_valgrind () {
 	fi
 }
 
+trace_level_=0
 want_trace () {
 	test "$trace" = t && {
 		test "$verbose" = t || test "$verbose_log" = t
@@ -895,7 +896,7 @@ want_trace () {
 test_eval_inner_ () {
 	# Do not add anything extra (including LF) after '$*'
 	eval "
-		want_trace && set -x
+		want_trace && trace_level_=$(($trace_level_+1)) && set -x
 		$*"
 }
 
@@ -926,7 +927,8 @@ test_eval_ () {
 		test_eval_ret_=$?
 		if want_trace
 		then
-			set +x
+			test 1 = $trace_level_ && set +x
+			trace_level_=$(($trace_level_-1))
 		fi
 	} 2>/dev/null 4>&2