Adds an entry to the index with Sha1 s and stage stage during a merge. If an entry already exists for this pathname/stage, it will be overwritten, otherwise it will be added if createEntry is true, and return an error if not. As a special case, if something is added as Stage0, then Stage1-3 entries
(c *Client, path IndexPath, mode EntryMode, s Sha1, stage Stage, size uint32, mtime int64, opts UpdateIndexOptions)
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