RFR: publish: add flag --follow

[Erik Helin]

Hi all,

please review this patch that adds the `--follow` switch and the `--browser` flag to `git publish`. These flags can be
used to automatically open a browser with the link that a remote host replies with in the response to the `git
push --set-upstream` command that `git publish` runs. The effect is a "poor mans" `git pr create` for those that can't
(or don't want) to have a personal access token on their machine.

For example, running `git publish --follow --browser=firefox` will open the "Open a pull request" page in Firefox.

Testing:
- [x] Manual testing of `git publish --follow --browser=firefox` on Linux x64

Thanks,
Erik

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

Commit messages:
 - publish: add flag --follow

Changes: https://git.openjdk.java.net/skara/pull/638/files
 Webrev: https://webrevs.openjdk.java.net/skara/638/webrev.00
  Stats: 100 lines in 1 file changed: 78 ins; 1 del; 21 mod
  Patch: https://git.openjdk.java.net/skara/pull/638.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/638/head:pull/638

PR: https://git.openjdk.java.net/skara/pull/638