commit: Return Modes in ListFilesResponse

NOTE: This previously was focused around returning OIDs because we were basing the client around a git-like filesystem implementation.

But now that we have switched to an OverlayFS-based approach, we only need Mode for now.

The code has not yet (as of 2022-06-22) been updated to reflect this change.

See more details at this comment: &7684 (comment 996791600)

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Description:

It is useful to have the modes exposed alongside of the path of each file for certain applications. This commit adds a modes field in the ListFilesResponse and populates it with the OID from the parser.

fixes: gitlab#355019 (closed)

Changelog: changed