cli/gitaly: Update storage flag default value

Justin Toblerrequested to merge
jt-cli-hooks into master

This change updates TestSetHooksSubcommand to improve its clarity by splitting command flags out onto separate lines and renaming error to expectedErr.

Currently the storage flag used by the hooks set command is set to "default". A better default would be to extract the storage name from the config. This change updates the storage flag default to use the config storage if there is only a single storage specified. If multiple storages are specified in the config and no flag provided an error is returned.

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