RFR: Add command git-publish

[Erik Helin]

Hi all,

this patch adds the command `git-publish` which is handy shortcut for pushing the current branch to the a remote (by default `origin`). I essentially grew tired of typing `git push -u origin <current-branch>` and don't want to set `git config --global push.default current` since then I don't get an upstream set.

## Testing
- Manual testing (pushed the branch for this PR with the tool)

Thanks,
Erik

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Commits:
 - 75a56601: Add git-publish to image
 - 21ae5f31: Add git-publish command

Changes: https://git.openjdk.java.net/skara/pull/198/files
 Webrev: https://webrevs.openjdk.java.net/skara/198/webrev.00
  Stats: 85 lines in 2 files changed: 84 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/198.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/198/head:pull/198

PR: https://git.openjdk.java.net/skara/pull/198