RFR: git-jcheck: add pre-push support

[Erik Helin]

Hi all,

please review this patch that adds support for the `--pre-push` and
`--setup-pre-push-hook` to `git-jcheck`. Setting up a pre-push hook can
beneficial for OpenJDK projects where contributors push directly (where they
don't use `git-pr` and pull requests).

Testing:
- Manual testing on Linux x64

Thanks,
Erik

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

Commits:
 - 87dd7851: git-jcheck: add pre-push support

Changes: https://git.openjdk.java.net/skara/pull/470/files
 Webrev: https://webrevs.openjdk.java.net/skara/470/webrev.00
  Stats: 65 lines in 1 file changed: 50 ins; 1 del; 14 mod
  Patch: https://git.openjdk.java.net/skara/pull/470.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/470/head:pull/470

PR: https://git.openjdk.java.net/skara/pull/470