RFR: git-webrev: add support for "upstream" remote

[Erik Helin]

Hi all,

please review this small patch that add supports to `git-webrev` for repositories with a remote named `upstream` (used for discovering the upstream repository).

Thanks,
Erik

## Testing
- [x] Manual testing of `git-webrev`

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Commits:
 - 5c078555: git-webrev: add support for "upstream" remote

Changes: https://git.openjdk.java.net/skara/pull/303/files
 Webrev: https://webrevs.openjdk.java.net/skara/303/webrev.00
  Stats: 27 lines in 1 file changed: 12 ins; 5 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/303.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/303/head:pull/303

PR: https://git.openjdk.java.net/skara/pull/303