submodule: add extension to encode gitdir paths

Add a submoduleEncoding extension which fixes filesystem collisions by
encoding gitdir paths. At a high level, this implements a mechanism to
encode -> validate -> retry until a working gitdir path is found.

Credit goes to Junio for coming up with this design: encoding is only
applied when necessary, e.g. uppercase characters are encoded only on
case-folding filesystems and only if a real conflict is detected.

To make this work, we rely on the submodule.<name>.gitdir config as the
single source of truth for gitidir paths: the config is always set when
the extension is enabled. Users who care about gitdir paths are expected
to get/set the config and not the underlying encoding implementation.

This commit adds the basic encoding logic which addresses nested gitdirs.
The next commit fixes case-folding, the next commit fixes names longer
than NAME_MAX. The idea is the encoding can be improved over time in a
way which is transparent to users.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Suggested-by: Phillip Wood <phillip.wood123@gmail.com>
Suggested-by: Patrick Steinhardt <ps@pks.im>
Based-on-patch-by: Brandon Williams <bwilliams.eng@gmail.com>
Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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