submodule: drop the top-level requirement

Use the new rev-parse --prefix option to process all paths given to the
submodule command, dropping the requirement that it be run from the
top-level of the repository.

Since the interpretation of a relative submodule URL depends on whether
or not "remote.origin.url" is configured, explicitly block relative URLs
in "git submodule add" when not at the top level of the working tree.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t7406-submodule-update.sh

@@ -80,6 +80,21 @@ test_expect_success 'submodule update detaching the HEAD ' '
 	)
 '
 
+test_expect_success 'submodule update from subdirectory' '
+	(cd super/submodule &&
+	 git reset --hard HEAD~1
+	) &&
+	mkdir super/sub &&
+	(cd super/sub &&
+	 (cd ../submodule &&
+	  compare_head
+	 ) &&
+	 git submodule update ../submodule &&
+	 cd ../submodule &&
+	 ! compare_head
+	)
+'
+
 apos="'";
 test_expect_success 'submodule update does not fetch already present commits' '
 	(cd submodule &&