[MacPorts] #59538: github portgroup doesn't work with default master_sites from other portgroups

[MacPorts]

#59538: github portgroup doesn't work with default master_sites from other
portgroups
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  Reporter:  jmroot  |      Owner:  (none)
      Type:  defect  |     Status:  new
  Priority:  Normal  |  Milestone:
 Component:  ports   |    Version:
Resolution:          |   Keywords:
      Port:          |
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Comment (by jmroot):

 The funny thing is, the documentation for the github portgroup is silent
 on whether it sets master_sites. The reader could infer that it does
 (because how else is something like github.tarball_from useful), but it's
 never actually stated. While Hyrum's Law is going to apply regardless,
 specifying more fully what portgroups are and aren't going to do for you
 might help.

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Ticket URL: <https://trac.macports.org/ticket/59538#comment:10>
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