RFR: cli: add command git-backport

[Sergey Bylokhov]

On Tue, 10 Nov 2020 14:12:52 GMT, Robin Westberg <rwestberg 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!

For some reason, I do not see backport command in the Skara help even on the freshly recloned version of skara, what could go wrong?

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