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

[Erik Helin]

> Hi all,
> 
> please review this patch that adds the `git pr csr` sub-command that corresponds to the
> [`/csr`](https://wiki.openjdk.java.net/display/SKARA/Pull+Request+Commands#PullRequestCommands-/csr) pull request
> command. A contributor who prefers the command-line can now state that a pull request requires a
> [CSR](https://wiki.openjdk.java.net/display/csr) by running `git pr csr --needed`.  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:

  Improve help message

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

Changes:
  - all: https://git.openjdk.java.net/skara/pull/702/files
  - new: https://git.openjdk.java.net/skara/pull/702/files/40f26ae3..a970e4ce

Webrevs:
 - full: https://webrevs.openjdk.java.net/skara/702/webrev.02
 - incr: https://webrevs.openjdk.java.net/skara/702/webrev.01-02

  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/skara/pull/702.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/702/head:pull/702

PR: https://git.openjdk.java.net/skara/pull/702