Merge branch 'hv/submodule-alt-odb' into maint

When a submodule repository uses alternate object store mechanism, some
commands that were started from the superproject did not notice it and
failed with "No such object" errors.  The subcommands of "git submodule"
command that recursed into the submodule in a separate process were OK;
only the ones that cheated and peeked directly into the submodule's
repository from the primary process were affected.

By Heiko Voigt
* hv/submodule-alt-odb:
  teach add_submodule_odb() to look for alternates

t/t4041-diff-submodule-option.sh

@@ -458,4 +458,38 @@ EOF
 	test_cmp expected actual
 '
 
+test_expect_success 'diff --submodule with objects referenced by alternates' '
+	mkdir sub_alt &&
+	(cd sub_alt &&
+		git init &&
+		echo a >a &&
+		git add a &&
+		git commit -m a
+	) &&
+	mkdir super &&
+	(cd super &&
+		git clone -s ../sub_alt sub &&
+		git init &&
+		git add sub &&
+		git commit -m "sub a"
+	) &&
+	(cd sub_alt &&
+		sha1_before=$(git rev-parse --short HEAD)
+		echo b >b &&
+		git add b &&
+		git commit -m b
+		sha1_after=$(git rev-parse --short HEAD)
+		echo "Submodule sub $sha1_before..$sha1_after:
+  > b" >../expected
+	) &&
+	(cd super &&
+		(cd sub &&
+			git fetch &&
+			git checkout origin/master
+		) &&
+		git diff --submodule > ../actual
+	)
+	test_cmp expected actual
+'
+
 test_done