RFR: git-pr: add summary sub-command [v4]

[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

Erik Helin has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains four commits:

 - Merge master
 - Add --remove switch
 - Show reply
 - 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.03
  Stats: 93 lines in 2 files changed: 93 ins; 0 del; 0 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