RFR: 316: git-webrev does not work without origin remote

[Erik Helin]

Hi all,

please review this patch that updates `git-webrev` to work with repositories
with slightly more unusual configurations by default. `git-webrev` will now by
default work with many more repositories that for example have multiple remotes,
repositories missing a remote named "origin", etc. For the most complicated
scenarios the user can now also specify `--remote` to pick the remote that
should be used for calculating outgoing changes.

Testing:
- Manual testing of `git-webrev` on Linux x64 with some more exotic repository
  configurations

Thanks,
Erik

-------------

Commit messages:
 - skara-316

Changes: https://git.openjdk.java.net/skara/pull/517/files
 Webrev: https://webrevs.openjdk.java.net/skara/517/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-316
  Stats: 48 lines in 1 file changed: 43 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/skara/pull/517.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/517/head:pull/517

PR: https://git.openjdk.java.net/skara/pull/517