RFR: git-pr: use upstream remote if available

[Erik Helin]

Hi all,

please review this PR that updates `git pr` to make use of a `upstream` remote
if one is available. This speeds things up a bit since we don't have use the
forges REST API's to find the upstream ("parent") repository.

Thanks,
Erik

- [x] Manual testing of `git-pr` on Linux x64

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Commits:
 - 3713cd63: git-pr: use upstream remote if available

Changes: https://git.openjdk.java.net/skara/pull/297/files
 Webrev: https://webrevs.openjdk.java.net/skara/297/webrev.00
  Stats: 26 lines in 1 file changed: 12 ins; 1 del; 13 mod
  Patch: https://git.openjdk.java.net/skara/pull/297.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/297/head:pull/297

PR: https://git.openjdk.java.net/skara/pull/297