git-submodule.sh: properly initialize shell variables

git-submodule inherits variables from the environment it is started in,
expects the internal variables init= and recursive= to have an empty
value, but doesn't initialize them appropriately.  Thanks to the
selftests, this can be reproduced through

 init=1 make test
 recursive=1 make test

With this commit the variables are initialized, and the selftests
succeed even if these variables have some values in the environment.

The bug was discovered through the Debian autobuilders
 http://bugs.debian.org/569594

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-submodule.sh

@@ -21,6 +21,8 @@ command=
 branch=
 reference=
 cached=
+recursive=
+init=
 files=
 nofetch=
 update=