submodule: fix case-folding gitdir filesystem collisions

submodule: fix case-folding gitdir filesystem collisions

Add a new check when extension.submodulePathConfig is enabled, to
detect and prevent case-folding filesystem colisions. When this
new check is triggered, a stricter casefolding aware URI encoding
is used to percent-encode uppercase characters.

By using this check/retry mechanism the uppercase encoding is
only applied when necessary, so case-sensitive filesystems are
not affected.

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