[MacPorts] #40076: libgit2: update to 0.19.0

[MacPorts]

#40076: libgit2: update to 0.19.0
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  Reporter:  nvenegas@…  |      Owner:  macports-tickets@…
      Type:  update      |     Status:  new
  Priority:  Normal      |  Milestone:
 Component:  ports       |    Version:
Resolution:              |   Keywords:  haspatch
      Port:  libgit2     |
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Comment (by nvenegas@…):

 Ah, good question: I couldn't find documentation on the
 "github.tarball_from" property, but when I tried "downloads" it 404'd when
 fetching 0.19.0. Looking at the URL it tried to download from, I noticed
 that https://github.com/libgit2/libgit2/releases links to
 https://github.com/libgit2/libgit2/archive/v0.19.0.tar.gz so I thought
 "archive" would be another valid value.

 It seemed to work, but now when I try my Portfile (using "archive") it
 fetches a binary from
 http://mse.uk.packages.macports.org/sites/packages.macports.org/libgit2

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Ticket URL: <https://trac.macports.org/ticket/40076#comment:3>
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