t: extend test_lazy_prereq

Allow test_lazy_prereq script to signal a programming error by
exiting with status 125 (like how bisect scripts do).  This is used
to signal a deprecated-and-then-removed prerequisite that should
never be used in tests anymore.

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

t/README

@@ -983,8 +983,10 @@ see test-lib-functions.sh for the full list and their options.
    SYMLINKS ...".  The script is run in a temporary directory inside
    a subshell, so you do not have to worry about removing temporary
    files you create there.  When the script exits with status 0, the
-   prerequisite is set.  Exiting with non-zero status makes the
+   prerequisite is set.  Exiting with non-zero status other than 125
-   prerequisite unsatisified.
+   makes the prerequisite unsatisified.  Exiting the script with 125
+   signals a programming error and is used to mark a prerequisite that
+   should not be used by test scripts.
 
  - test_expect_code <exit-code> <command>
 

t/test-lib-functions.sh

@@ -773,6 +773,8 @@ mkdir -p "$TRASH_DIRECTORY/prereq-test-dir-'"$1"'" &&
 	rm -rf "$TRASH_DIRECTORY/prereq-test-dir-$1"
 	if test "$eval_ret" = 0; then
 		say >&3 "prerequisite $1 ok"
+	elif test "$eval_ret" = 125; then
+		:;
 	else
 		say >&3 "prerequisite $1 not satisfied"
 	fi
@@ -811,6 +813,9 @@ test_have_prereq () {
 				if test_run_lazy_prereq_ "$prerequisite" "$script"
 				then
 					test_set_prereq $prerequisite
+				elif test $? = 125
+				then
+					BUG "Do not use $prerequisite"
 				fi
 				lazily_tested_prereq="$lazily_tested_prereq$prerequisite "
 			esac