patch 8.0.1489: there is no easy way to get the global directory

Problem:    There is no easy way to get the global directory, esp. if some
            windows have a local directory.
Solution:   Make getcwd(-1) return the global directory. (Andy Massimino,
            closes #2606)
diff --git a/src/testdir/test_getcwd.vim b/src/testdir/test_getcwd.vim
index 15eab2a..334b890 100644
--- a/src/testdir/test_getcwd.vim
+++ b/src/testdir/test_getcwd.vim
@@ -37,6 +37,7 @@
 	new
 	call mkdir('Xtopdir')
 	cd Xtopdir
+	let g:topdir = getcwd()
 	call mkdir('Xdir1')
 	call mkdir('Xdir2')
 	call mkdir('Xdir3')
@@ -56,36 +57,44 @@
 	3wincmd w
 	lcd Xdir1
 	call assert_equal("a Xdir1 1", GetCwdInfo(0, 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	wincmd W
 	call assert_equal("b Xtopdir 0", GetCwdInfo(0, 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	wincmd W
 	lcd Xdir3
 	call assert_equal("c Xdir3 1", GetCwdInfo(0, 0))
 	call assert_equal("a Xdir1 1", GetCwdInfo(bufwinnr("a"), 0))
 	call assert_equal("b Xtopdir 0", GetCwdInfo(bufwinnr("b"), 0))
 	call assert_equal("c Xdir3 1", GetCwdInfo(bufwinnr("c"), 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	wincmd W
 	call assert_equal("a Xdir1 1", GetCwdInfo(bufwinnr("a"), tabpagenr()))
 	call assert_equal("b Xtopdir 0", GetCwdInfo(bufwinnr("b"), tabpagenr()))
 	call assert_equal("c Xdir3 1", GetCwdInfo(bufwinnr("c"), tabpagenr()))
+	call assert_equal(g:topdir, getcwd(-1))
 
 	tabnew x
 	new y
 	new z
 	3wincmd w
 	call assert_equal("x Xtopdir 0", GetCwdInfo(0, 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	wincmd W
 	lcd Xdir2
 	call assert_equal("y Xdir2 1", GetCwdInfo(0, 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	wincmd W
 	lcd Xdir3
 	call assert_equal("z Xdir3 1", GetCwdInfo(0, 0))
 	call assert_equal("x Xtopdir 0", GetCwdInfo(bufwinnr("x"), 0))
 	call assert_equal("y Xdir2 1", GetCwdInfo(bufwinnr("y"), 0))
 	call assert_equal("z Xdir3 1", GetCwdInfo(bufwinnr("z"), 0))
+	call assert_equal(g:topdir, getcwd(-1))
 	let tp_nr = tabpagenr()
 	tabrewind
 	call assert_equal("x Xtopdir 0", GetCwdInfo(3, tp_nr))
 	call assert_equal("y Xdir2 1", GetCwdInfo(2, tp_nr))
 	call assert_equal("z Xdir3 1", GetCwdInfo(1, tp_nr))
+	call assert_equal(g:topdir, getcwd(-1))
 endfunc