RFR: 210: git-pr: add BRANCH column to git-pr list

[Erik Helin]

Hi all,

please review this patch that adds the "BRANCH" column to the `git pr list`
output. The branch column will list the local branches in a repository that
corresponds to a pull request.

@rwestberg I had to make a few changes to `PullRequest`:

- renamed `sourceRef` to `fetchRef`
- added `sourceRef` which now really means the reference in the source
  repository ��
- added `sourceRepository` for getting the source repository for a pull request

I did the rename of `sourceRef` to `fetchRef` first, so I shouldn't have missed
updating any places where it was used.

Thanks,
Erik

## Testing
- [x] `make test` on Linux x64
- [x] Manual testing of `git pr list` on Linux x64

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

Commits:
 - 90ff1737: skara-210

Changes: https://git.openjdk.java.net/skara/pull/338/files
 Webrev: https://webrevs.openjdk.java.net/skara/338/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-210
  Stats: 70 lines in 8 files changed: 62 ins; 0 del; 8 mod
  Patch: https://git.openjdk.java.net/skara/pull/338.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/338/head:pull/338

PR: https://git.openjdk.java.net/skara/pull/338