[PATCH] test-hashmap: document why it is no longer used but still there

[Junio C Hamano <gitster@pobox.com>]

As I ended up wasting a few dozen minutes looking for the reason why
this is still here, help future developers by saving them from
wasting their time by documenting why this code that apparently is
not used by anybody is still here.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 t/helper/test-hashmap.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git c/t/helper/test-hashmap.c w/t/helper/test-hashmap.c
index 7782ae585e..16a3145c3a 100644
--- c/t/helper/test-hashmap.c
+++ w/t/helper/test-hashmap.c
@@ -137,6 +137,11 @@ static void perf_hashmap(unsigned int method, unsigned int rounds)
  * Read stdin line by line and print result of commands to stdout:
  *
  * perfhashmap method rounds -> test hashmap.[ch] performance
+ *
+ * NOTE: this is not used by any of our mechanized build & test
+ * procedure, after 3469a236 (t: port helper/test-hashmap.c to
+ * unit-tests/t-hashmap.c, 2024-08-03).  See the log message of that
+ * commit for the reason why this is still here.
  */
 int cmd__hashmap(int argc UNUSED, const char **argv UNUSED)
 {