[BUG] ls-files '**' globstar matches one or more directories instead of zero or more directories

To reproduce, go to any git repository and run:

    diff <(git ls-files '**/*' | sort) <(git ls-files | sort)

Expected result: No output since both commands should produce identical output.

Actual result: '**/*' only matches files at least one directory deep.

The same happens with eg '**/Makefile' - only Makefiles in subdirectories are listed.

I have personally tested with 2.7.8 and 2.12.0. Others on IRC report that this happens in the next branch.