RFR: git-pr: add summary sub-command [v2]

[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 incrementally with one additional commit since the last revision:

  Show reply

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/700/files
  - new: https://git.openjdk.java.net/skara/pull/700/files/ea8a9791..bcaf0e78

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/700/webrev.01
 - incr: https://webrevs.openjdk.java.net/skara/700/webrev.00-01

  Stats: 1 line in 1 file changed: 1 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