pkgbase: pass --recurse-submodules to git ls-files

When generating source packages. Although submodules are not used by
FreeBSD itself they may be used by downstream projects. By default
for submodules git ls-files just emits the submodule directory name,
which resulted in:

pkg: pkg_checksum_hash_sha256_file(read failed): Is a directory

Passing --recurse-submodules lists all of the files in each submodule
(which is desired when submodules are in use), and has no effect when
submodules are not present.

Reviewed by: bapt, manu
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D42983

(cherry picked from commit 3b1904d9eb0468a49be3cd1d97de6d7ecaa66a43)