re-revert tests: add a special setup where stash.useBuiltin is off

We temporarily reverted part of this commit to allow merging `add-p`
without conflicts. Now it is time to re-apply that part.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

t/README

@@ -384,6 +384,10 @@ the --no-sparse command-line argument.
 GIT_TEST_PRELOAD_INDEX=<boolean> exercises the preload-index code path
 by overriding the minimum number of cache entries required per thread.
 
+GIT_TEST_STASH_USE_BUILTIN=<boolean>, when false, disables the
+built-in version of git-stash. See 'stash.useBuiltin' in
+git-config(1).
+
 GIT_TEST_ADD_I_USE_BUILTIN=<boolean>, when true, enables the
 builtin version of git add -i. See 'add.interactive.useBuiltin' in
 git-config(1).