RFR: cli: add command git-backport

[Erik Helin]

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

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Commit messages:
 - cli: add command git-backport

Changes: https://git.openjdk.java.net/skara/pull/948/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=948&range=00
  Stats: 220 lines in 2 files changed: 219 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/948.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/948/head:pull/948

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