tests: skip test_eval_ in internal chain-lint

To check for broken &&-chains, we run "fail_117 && $1" as a test
snippet, and check the exit code. We use test_eval_ to do so, because
that's the way we run the actual test.

But we don't need any of its niceties, like "set -x" tracing. In fact,
they hinder us, because we have to explicitly disable them. So let's
skip that and use "eval" more directly, which is simpler. I had hoped it
would also be faster, but it doesn't seem to produce a measurable
improvement (probably because it's just running internal shell commands,
with no subshells or forks).

Note that there is one gotcha: even though we don't intend to run any of
the commands if the &&-chain is intact, an error like this:

   test_expect_success 'broken' '
	# this next line breaks the &&-chain
	true
	# and then this one is executed even by the linter
	return 1
   '

means we'll "return 1" from the eval, and thus from test_run_(). We
actually do notice this in test_expect_success, but only by saying "hey,
this test didn't say it was OK, so it must have failed", which is not
right (it should say "broken &&-chain").

We can handle this by calling test_eval_inner_() instead, which is our
trick for wrapping "return" in a test snippet. But to do that, we have
to push the trace code out of that inner function and into test_eval_().
This is arguably where it belonged in the first place, but it never
mattered because the "inner_" function had only one caller.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -1041,10 +1041,7 @@ want_trace () {
 # (and we want to make sure we run any cleanup like
 # "set +x").
 test_eval_inner_ () {
-	# Do not add anything extra (including LF) after '$*'
-	eval "
-		want_trace && trace_level_=$(($trace_level_+1)) && set -x
-		$*"
+	eval "$*"
 }
 
 test_eval_ () {
@@ -1069,7 +1066,10 @@ test_eval_ () {
 	#     be _inside_ the block to avoid polluting the "set -x" output
 	#
 
-	test_eval_inner_ "$@" </dev/null >&3 2>&4
+	# Do not add anything extra (including LF) after '$*'
+	test_eval_inner_ </dev/null >&3 2>&4 "
+		want_trace && trace_level_=$(($trace_level_+1)) && set -x
+		$*"
 	{
 		test_eval_ret_=$?
 		if want_trace
@@ -1095,18 +1095,13 @@ test_run_ () {
 	expecting_failure=$2
 
 	if test "${GIT_TEST_CHAIN_LINT:-1}" != 0; then
-		# turn off tracing for this test-eval, as it simply creates
-		# confusing noise in the "-x" output
-		trace_tmp=$trace
-		trace=
 		# 117 is magic because it is unlikely to match the exit
 		# code of other programs
-		test_eval_ "fail_117 && $1"
+		test_eval_inner_ "fail_117 && $1" </dev/null >&3 2>&4
 		if test $? != 117
 		then
 			BUG "broken &&-chain: $1"
 		fi
-		trace=$trace_tmp
 	fi
 
 	setup_malloc_check