RFR: git-pr: add summary sub-command

[Erik Helin]

Hi all,

please review this patch that adds the `git pr summary` sub-command which corresponds to the
[`/summary`](https://wiki.openjdk.java.net/display/SKARA/Pull+Request+Commands#PullRequestCommands-/summary) pull
request command. Contributors who prefer to use the command-line can now run `git pr summary` which will open their
configured editor for writing a summary.

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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

Commit messages:
 - git-pr: add summary sub-command

Changes: https://git.openjdk.java.net/skara/pull/700/files
 Webrev: https://webrevs.openjdk.java.net/skara/700/webrev.00
  Stats: 84 lines in 2 files changed: 83 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/700.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/700/head:pull/700

PR: https://git.openjdk.java.net/skara/pull/700