[MacPorts] #58097: carthage @0.31.2: add shell completions

[MacPorts]

#58097: carthage @0.31.2: add shell completions
---------------------------+-----------------------
 Reporter:  FranklinYu     |      Owner:  saagarjha
     Type:  enhancement    |     Status:  assigned
 Priority:  Normal         |  Milestone:
Component:  ports          |    Version:
 Keywords:  bash zsh fish  |       Port:  carthage
---------------------------+-----------------------
 It was mentioned in #57923 that the
 [https://github.com/Carthage/Carthage/tree/0.31.2/Source/Scripts
 Bash/Zsh/Fish shell completions] are missing in the port. In contrast,
 they are included in [https://github.com/Homebrew/homebrew-
 core/blob/d7df4375a5fcba6f8d269df8e7b04184ffaf3b85/Formula/carthage.rb#L20-L22
 Homebrew package].

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Ticket URL: <https://trac.macports.org/ticket/58097>
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