Teach read-tree the -n|--dry-run option

The option can be used to check if read-tree with the same set of other
options like "-m" and "-u" would succeed without actually changing either
the index or the working tree.

The relevant tests in the t10?? range were extended to do a read-tree -n
before the real read-tree to make sure neither the index nor any local
files were changed with -n and the same exit code as without -n is
returned. The helper functions added for that purpose reside in the new
t/lib-read-tree.sh file.

The only exception is #13 in t1004 ("unlinking an un-unlink-able
symlink"). As this is an issue of wrong directory permissions it is not
detected with -n.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t1008-read-tree-overlay.sh b/t/t1008-read-tree-overlay.sh
index f9e0028..4c50ed9 100755
--- a/t/t1008-read-tree-overlay.sh
+++ b/t/t1008-read-tree-overlay.sh
@@ -3,6 +3,7 @@
 test_description='test multi-tree read-tree without merging'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-read-tree.sh
 
 test_expect_success setup '
 	echo one >a &&
@@ -21,7 +22,7 @@
 '
 
 test_expect_success 'multi-read' '
-	git read-tree initial master side &&
+	read_tree_must_succeed initial master side &&
 	(echo a; echo b/c) >expect &&
 	git ls-files >actual &&
 	test_cmp expect actual