submodule add: extend force flag to add existing repos

Currently the force flag in `git submodule add` takes care of possibly
ignored files or when a name collision occurs.

However there is another situation where submodule add comes in handy:
When you already have a gitlink recorded, but no configuration was
done (i.e. no .gitmodules file nor any entry in .git/config) and you
want to generate these config entries. For this situation allow
`git submodule add` to proceed if there is already a submodule at the
given path in the index.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -207,8 +207,14 @@ cmd_add()
 			tstart
 			s|/*$||
 		')
-	git ls-files --error-unmatch "$sm_path" > /dev/null 2>&1 &&
-	die "$(eval_gettext "'\$sm_path' already exists in the index")"
+	if test -z "$force"
+	then
+		git ls-files --error-unmatch "$sm_path" > /dev/null 2>&1 &&
+		die "$(eval_gettext "'\$sm_path' already exists in the index")"
+	else
+		git ls-files -s "$sm_path" | sane_grep -v "^160000" > /dev/null 2>&1 &&
+		die "$(eval_gettext "'\$sm_path' already exists in the index and is not a submodule")"
+	fi
 
 	if test -z "$force" && ! git add --dry-run --ignore-missing "$sm_path" > /dev/null 2>&1
 	then

t/t7400-submodule-basic.sh

@@ -152,6 +152,20 @@ test_expect_success 'submodule add to .gitignored path with --force' '
 	)
 '
 
+test_expect_success 'submodule add to reconfigure existing submodule with --force' '
+	(
+		cd addtest-ignore &&
+		git submodule add --force bogus-url submod &&
+		git submodule add -b initial "$submodurl" submod-branch &&
+		test "bogus-url" = "$(git config -f .gitmodules submodule.submod.url)" &&
+		test "bogus-url" = "$(git config submodule.submod.url)" &&
+		# Restore the url
+		git submodule add --force "$submodurl" submod
+		test "$submodurl" = "$(git config -f .gitmodules submodule.submod.url)" &&
+		test "$submodurl" = "$(git config submodule.submod.url)"
+	)
+'
+
 test_expect_success 'submodule add --branch' '
 	echo "refs/heads/initial" >expect-head &&
 	cat <<-\EOF >expect-heads &&