[MacPorts] #61050: Add `port vercmp` command

[MacPorts]

#61050: Add `port vercmp` command
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  Reporter:  ryandesign   |      Owner:  (none)
      Type:  enhancement  |     Status:  new
  Priority:  Normal       |  Milestone:
 Component:  base         |    Version:  2.6.99
Resolution:               |   Keywords:
      Port:               |
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Comment (by kurthindenburg):

 IMHO the real benefit is that we could add a section to the guide for
 epoch saying if 'port vercmp old new' is less then you need to add epoch 1
 to the portfile.

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Ticket URL: <https://trac.macports.org/ticket/61050#comment:4>
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