stash apply: report status correctly even in a worktree's subdirectory

When Git wants to spawn a child Git process inside a worktree's
subdirectory while `GIT_DIR` is set, we need to take care of specifying
the work tree's top-level directory explicitly because it cannot be
discovered: the current directory is _not_ the top-level directory of
the work tree, and neither is it inside the parent directory of
`GIT_DIR`.

This fixes the problem where `git stash apply` would report pretty much
everything deleted or untracked when run inside a worktree's
subdirectory.

To make sure that we do not introduce the "reverse problem", i.e. when
`GIT_WORK_TREE` is defined but `GIT_DIR` is not, we simply make sure
that both are set.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -497,6 +497,10 @@ static int do_apply_stash(const char *prefix, struct stash_info *info,
 		 */
 		cp.git_cmd = 1;
 		cp.dir = prefix;
+		argv_array_pushf(&cp.env_array, GIT_WORK_TREE_ENVIRONMENT"=%s",
+				 absolute_path(get_git_work_tree()));
+		argv_array_pushf(&cp.env_array, GIT_DIR_ENVIRONMENT"=%s",
+				 absolute_path(get_git_dir()));
 		argv_array_push(&cp.args, "status");
 		run_command(&cp);
 	}

t/t3908-stash-in-worktree.sh

@@ -0,0 +1,27 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Johannes E Schindelin
+#
+
+test_description='Test git stash in a worktree'
+
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	test_commit initial &&
+	git worktree add wt &&
+	test_commit -C wt in-worktree
+'
+
+test_expect_success 'apply in subdirectory' '
+	mkdir wt/subdir &&
+	(
+		cd wt/subdir &&
+		echo modified >../initial.t &&
+		git stash &&
+		git stash apply >out
+	) &&
+	grep "\.\.\/initial\.t" wt/subdir/out
+'
+
+test_done