RFR: git-pr: add csr sub-command

[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

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

Commit messages:
 - Add csr to GitPr.java
 - git-pr: add csr sub-command

Changes: https://git.openjdk.java.net/skara/pull/702/files
 Webrev: https://webrevs.openjdk.java.net/skara/702/webrev.00
  Stats: 104 lines in 4 files changed: 99 ins; 0 del; 5 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