submodule: hash the submodule name for the gitdir path

submodule: hash the submodule name for the gitdir path

If none of the previous plain-text / encoding / derivation steps work
and case 2.4 is reached, then try a hash of the submodule name to see
if that can be a valid gitdir before giving up and throwing an error.

This is a "last resort" type of measure to avoid conflicts since it
loses the human readability of the gitdir path. This logic will be
reached in rare cases, as can be seen in the test we added.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>