submodules.cmake: don't use "--depth 1" with old Git

Old Git may not work with "--depth 1" when the referenced commit hash
is far from HEAD.

Newer Git improves the situation. For example:
https://github.com/git/git/commit/fb43e31f2b43076e7a30c9cd00d0241cb8cf97eb

It's safe to not use "--depth 1" with old Git.

Closes #2049