RFR: git-pr: add summary sub-command

[Erik Helin]

On Thu, 9 Jul 2020 10:16:49 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> 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

/summary
Adds the `git pr summary` sub-command corresponding to the `/summary` pull
request command.

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