RFR: git-pr: add multiple new options to "git pr set"

[Erik Helin]

Hi all,

please review this patch that adds a number of options to `git pr set`:

- `--open` - set the pull request's state to "open"
- `--closed` - set the pull request's state to "closed"
- `--title` - set the pull request's title
- `--body` - set the pull request's body

Already existing options to `git pr set` include:

- `--assignees` - set the assignees of the pull request
- `--no-draft` - set the pull request's state to "not draft"

All in all, `git pr set` has now become rather useful ��

Testing:
- Manual testing of `git pr set` on Linux x64

Thanks,
Erik

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

Commits:
 - 4732856b: git-pr: add options to "git pr set"

Changes: https://git.openjdk.java.net/skara/pull/461/files
 Webrev: https://webrevs.openjdk.java.net/skara/461/webrev.00
  Stats: 49 lines in 1 file changed: 49 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/461.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/461/head:pull/461

PR: https://git.openjdk.java.net/skara/pull/461