RFR: git-pr: add summary sub-command [v3]

[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:

  Add --remove switch

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/700/files
  - new: https://git.openjdk.java.net/skara/pull/700/files/bcaf0e78..56e36cf1

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/700/webrev.02
 - incr: https://webrevs.openjdk.java.net/skara/700/webrev.01-02

  Stats: 9 lines in 1 file changed: 9 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