preload-index: update GIT_FORCE_PRELOAD_TEST support

Rename GIT_FORCE_PRELOAD_TEST to GIT_TEST_PRELOAD_INDEX for consistency with
the other GIT_TEST_ special setups and properly document its use.

Add logic in t/test-lib.sh to give a warning when the old variable is set to
let people know they need to update their environment to use the new
variable.

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

preload-index.c

@@ -85,7 +85,7 @@ static void preload_index(struct index_state *index,
 		return;
 
 	threads = index->cache_nr / THREAD_COST;
-	if ((index->cache_nr > 1) && (threads < 2) && git_env_bool("GIT_FORCE_PRELOAD_TEST", 0))
+	if ((index->cache_nr > 1) && (threads < 2) && git_env_bool("GIT_TEST_PRELOAD_INDEX", 0))
 		threads = 2;
 	if (threads < 2)
 		return;

t/README

@@ -327,6 +327,9 @@ GIT_TEST_INDEX_VERSION=<n> exercises the index read/write code path
 for the index version specified.  Can be set to any valid version
 (currently 2, 3, or 4).
 
+GIT_TEST_PRELOAD_INDEX=<boolean> exercises the preload-index code path
+by overriding the minimum number of cache entries required per thread.
+
 Naming Tests
 ------------
 

t/t7519-status-fsmonitor.sh

@@ -238,9 +238,9 @@ do
 		git config core.preloadIndex $preload_val &&
 		if test $preload_val = true
 		then
-			GIT_FORCE_PRELOAD_TEST=$preload_val; export GIT_FORCE_PRELOAD_TEST
+			GIT_TEST_PRELOAD_INDEX=$preload_val; export GIT_TEST_PRELOAD_INDEX
 		else
-			unset GIT_FORCE_PRELOAD_TEST
+			sane_unset GIT_TEST_PRELOAD_INDEX
 		fi
 	'
 

t/test-lib.sh

@@ -154,6 +154,7 @@ check_var_migration () {
 
 check_var_migration GIT_FSMONITOR_TEST GIT_TEST_FSMONITOR
 check_var_migration TEST_GIT_INDEX_VERSION GIT_TEST_INDEX_VERSION
+check_var_migration GIT_FORCE_PRELOAD_TEST GIT_TEST_PRELOAD_INDEX
 
 # Use specific version of the index file format
 if test -n "${GIT_TEST_INDEX_VERSION:+isset}"