Commit b119a687d4 (test-lib: ignore leaks in the sanitizer's thread
code, 2025-01-01) added code to suppress a false positive in the leak
checker. But if you're just reading the code, the obscure grep call is a
bit of a head-scratcher. Let's add a brief comment explaining what's
going on (and anybody digging further can find this commit or that one
for all the details).

Signed-off-by: Jeff King <redacted>
---
 t/test-lib.sh | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/t/test-lib.sh b/t/test-lib.sh
index 898c2267b8..9f27a49995 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh

@@ -1172,6 +1172,11 @@ test_atexit_handler () {
 check_test_results_san_file_has_entries_ () {
 	test -z "$TEST_RESULTS_SAN_FILE" && return 1
 
+	# Lines marked with DEDUP_TOKEN show unique leaks. We only care that we
+	# found at least one.
+	#
+	# But also suppress any false positives caused by bugs or races in the
+	# sanitizer itself.
 	grep -s ^DEDUP_TOKEN "$TEST_RESULTS_SAN_FILE".* |
 	grep -qv sanitizer::GetThreadStackTopAndBottom
 }