recursive submodules: test for relative paths

recursive submodules: test for relative paths

"git submodule update --init --recursive" uses full path to refer to
the true location of the repository in the "gitdir:" pointer for
nested submodules; the command used to use relative paths.

This was reported by Norio Nomura in $gmane/290280.

The root cause for that bug is in using recursive submodules as
their relative path handling was broken in ee8838d (2015-09-08,
submodule: rewrite `module_clone` shell function in C).

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