test: caution on our version of 'yes'

During a review of a patch, we noticed that we use our own imitation
of 'yes' with the limit of 99 lines.  It is very tempting to lift this
arbitrary limit, but the limit is there for a reason.

Add an in-code comment to prevent future developers from wasting
their time.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/t/README b/t/README
index 1326fd7505..f4e1a82657 100644
--- a/t/README
+++ b/t/README
@@ -927,6 +927,15 @@ library for your script to use.
    test_oid_init or test_oid_cache.  Providing an unknown key is an
    error.
 
+ - yes [<string>]
+
+   This is often seen in modern UNIX but some platforms lack it, so
+   the test harness overrides the platform implementation with a
+   more limited one.  Use this only when feeding a handful lines of
+   output to the downstream---unlike the real version, it generates
+   only up to 99 lines.
+
+
 Prerequisites
 -------------