submodule--helper: fix filesystem collisions by encoding gitdir paths

submodule--helper: fix filesystem collisions by encoding gitdir paths

Fix nested filesystem collisions by url-encoding gitdir paths stored
in submodule.%s.gitdir, when extensions.submodulePathConfig is enabled.

Credit goes to Junio and Patrick for coming up with this design: the
encoding is only applied when necessary, to newly added submodules.

Existing modules don't need the encoding because git already errors
out when detecting nested gitdirs before this patch.

This commit adds the basic url-encoding and some tests. Next commits
extend the encode -> validate -> retry loop to fix more conflicts.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>