submodule tests: reset "trace.out" between "grep" invocations

Fix test patterns added in 62104ba14a (submodules: allow parallel
fetching, add tests and documentation, 2015-12-15) and
a028a1930c (fetching submodules: respect `submodule.fetchJobs` config
option, 2016-02-29).

In the former case we were leaving a trace.out file at the top-level
for any subsequent tests (there are none, currently). Let's clean the
file up instead.

In the latter case we were testing that a given configuration would
result in "N tasks" in the log, but we were grepping through the log
for all previous such tests, when we really meant to clear the logs
between the "grep" invocations.

In practice this resulted in no logic error, as e.g. "--fetch 7" would
not print out a "9 tasks" line, but let's be paranoid and stop
implicitly assuming that that's the case.

This change was originally left out of 51243f9f0f (run-command API:
don't fall back on online_cpus(), 2022-10-12), which added the
">trace.out" seen at the end of the context.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

t/t5526-fetch-submodules.sh

@@ -178,6 +178,7 @@ test_expect_success "submodule.recurse option triggers recursive fetch" '
 '
 
 test_expect_success "fetch --recurse-submodules -j2 has the same output behaviour" '
+	test_when_finished "rm -f trace.out" &&
 	add_submodule_commits &&
 	(
 		cd downstream &&
@@ -705,15 +706,22 @@ test_expect_success "'fetch.recurseSubmodules=on-demand' works also without .git
 
 test_expect_success 'fetching submodules respects parallel settings' '
 	git config fetch.recurseSubmodules true &&
+	test_when_finished "rm -f downstream/trace.out" &&
 	(
 		cd downstream &&
 		GIT_TRACE=$(pwd)/trace.out git fetch &&
 		grep "1 tasks" trace.out &&
+		>trace.out &&
+
 		GIT_TRACE=$(pwd)/trace.out git fetch --jobs 7 &&
 		grep "7 tasks" trace.out &&
+		>trace.out &&
+
 		git config submodule.fetchJobs 8 &&
 		GIT_TRACE=$(pwd)/trace.out git fetch &&
 		grep "8 tasks" trace.out &&
+		>trace.out &&
+
 		GIT_TRACE=$(pwd)/trace.out git fetch --jobs 9 &&
 		grep "9 tasks" trace.out &&
 		>trace.out &&