[MacPorts] #41868: libgit2 @0.17.0: update to 0.20.0

[MacPorts]

#41868: libgit2 @0.17.0: update to 0.20.0
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  Reporter:  juanrgar@…  |      Owner:  macports-tickets@…
      Type:  update      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:  haspatch
      Port:  libgit2     |
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Comment (by ryandesign@…):

 Fetch fails. The port uses "github.tarball_from downloads" but the
 downloads feature is deprecated and no new "downloads" can be created, and
 no "download" exists for 0.20.0. "Tags" and "releases" do exist for that
 version. The github portgroup supports "tags", but not yet "releases".

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Ticket URL: <https://trac.macports.org/ticket/41868#comment:1>
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