RFR: git-webrev: add --json flag [v2]

[Erik Helin]

> Hi all,
> 
> please review this patch that adds the `--json` flag to `git-webrev`. The `--json` flag is not meant to be used by a
> humans, it generates a JSON version of a "webrev" that tools can consume (in comparison to HTML that is usually
> generated).  Testing:
> - [x] Manual testing on Linux x64
> 
> Thanks,
> Erik

Erik Helin has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains two commits:

 - Merge branch 'master'
 - git-webrev: add --json flag

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

Changes: https://git.openjdk.java.net/skara/pull/717/files
 Webrev: https://webrevs.openjdk.java.net/skara/717/webrev.01
  Stats: 283 lines in 6 files changed: 252 ins; 5 del; 26 mod
  Patch: https://git.openjdk.java.net/skara/pull/717.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/717/head:pull/717

PR: https://git.openjdk.java.net/skara/pull/717