Merge pull request git-for-windows#2119 from dscho/update-stash-to-current

Update the built-in `git stash` to the latest version

Author: dscho

t/README

@@ -388,6 +388,10 @@ 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_STASH_USE_BUILTIN=<boolean>, when false, disables the
+built-in version of git-stash. See 'stash.useBuiltin' in
+git-config(1).
+
 GIT_TEST_INDEX_THREADS=<n> enables exercising the multi-threaded loading
 of the index for the whole test suite by bypassing the default number of
 cache entries and thread minimums. Setting this to 1 will make the