RFR: cli: add command git-backport

[Robin Westberg]

On Mon, 9 Nov 2020 19:46:57 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that adds the new CLI command `git backport`. A contributing backporting a commit can now simply run `git backport --from=jdk <HASH>` to backport a commit from the JDK repo targeting the current repository. The "other" direction is also possible, where the user runs the command in a local clone of their personal fork of the jdk repository: `git backport --to=jdk16u <HASH>`. The user can optionally supply a branch, as in `git backport --to=jfx --branch=jfx14 <HASH>`.
> 
> Behind the scenes the `git backport` do nothing more than just making a "backport" commit comment on the given hash.
> 
> Testing:
> - [x] Manual testing on Linux x64
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/948