[MacPorts] #54605: Update guide to tell user where to get "original" Portfile for patching

[MacPorts]

#54605: Update guide to tell user where to get "original" Portfile for patching
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  Reporter:  jfcaron3  |      Owner:
      Type:  defect    |     Status:  new
  Priority:  Normal    |  Milestone:
 Component:  guide     |    Version:
Resolution:            |   Keywords:
      Port:            |
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Comment (by kencu):

 I find your best bet is to sudo copy the original Portfile in it's
 original location to `Portfile.orig`, make your edits to `Portfile`, and
 then diff them with `diff -u Portfile.orig Portfile >
 ~/Desktop/somename.diff`

 Or go the github way instead, if you're going to do this at all
 frequently.

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Ticket URL: <https://trac.macports.org/ticket/54605#comment:1>
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