read-cache.c: make $GIT_TEST_SPLIT_INDEX boolean

While at there, document about this special mode when running the test
suite.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

ci/run-build-and-tests.sh

@@ -11,7 +11,7 @@ make --jobs=2
 make --quiet test
 if test "$jobname" = "linux-gcc"
 then
-	GIT_TEST_SPLIT_INDEX=YesPlease make --quiet test
+	GIT_TEST_SPLIT_INDEX=yes make --quiet test
 fi
 
 check_unignored_build_artifacts

read-cache.c

@@ -2268,7 +2268,7 @@ static int do_write_index(struct index_state *istate, struct tempfile *tempfile,
 
 	if (!istate->version) {
 		istate->version = get_index_format_default();
-		if (getenv("GIT_TEST_SPLIT_INDEX"))
+		if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0))
 			init_split_index(istate);
 	}
 
@@ -2559,7 +2559,7 @@ int write_locked_index(struct index_state *istate, struct lock_file *lock,
 		goto out;
 	}
 
-	if (getenv("GIT_TEST_SPLIT_INDEX")) {
+	if (git_env_bool("GIT_TEST_SPLIT_INDEX", 0)) {
 		int v = si->base_sha1[0];
 		if ((v & 15) < 6)
 			istate->cache_changed |= SPLIT_INDEX_ORDERED;

t/README

@@ -293,6 +293,17 @@ and know what setup is needed for it.  Or when you want to run
 everything up to a certain test.
 
 
+Running tests with special setups
+---------------------------------
+
+The whole test suite could be run to test some special features
+that cannot be easily covered by a few specific test cases. These
+could be enabled by running the test suite with correct GIT_TEST_
+environment set.
+
+GIT_TEST_SPLIT_INDEX=<boolean> forces split-index mode on the whole
+test suite. Accept any boolean values that are accepted by git-config.
+
 Naming Tests
 ------------