[skara] RFR: cli: rework git-backport

[Erik Helin]

Hi all,

please review this patch that reworks how `git-backport` functions. `git-backport` is now much smaller (and more focused) than it used to be and now composes much better with other Skara CLI tools like `git-pr-create`. `git-backport` now simply fetches a commit from a remote repository and cherry-picks the fetched commit on top of the current branch (without committing). `git-backport` then finally makes a commit with the proper backport commit message (a commit message of the form `Backport <HASH>`).

The tool `git-backport` is now meant to be used in combination with `git-pr-create`, `git-pr-set` and `git-pr-integrate`. The following example shows how a commit can be backported, a pull request created, marking the pull request as clean and then finally integrating the pull request:

$ git checkout -b backport-5a01c3d68ac2
$ git backport --from=openjdk/jdk 5a01c3d68ac22b7ee6f0746405a9bdef43281cb7
$ git pr create
$ sleep 15 # give the bots some time to work
$ git pr set --clean
$ sleep 15 # allow the bots to do some more work
$ git pr integrate
$ git checkout -

All (or parts) of the above can of course be wrapped into a [git alias](https://git-scm.com/book/en/v2/Git-Basics-Git-Aliases) for those that don't like to type. The following show an example alias defined in `~/.gitconfig`:

[alias]
bp = "!f() { \
        git checkout -b backport-$1 && \
        git backport --from=openjdk/jdk $1 && \
        git pr create; \
      }; f"

A user can then simply run `git bp 5a01c3d68ac22b7ee6f0746405a9bdef43281cb7`.

The `--from` parameter to `git-backport` is configurable, so a user who most likely often backports from [openjdk/jdk](https://github.com/openjdk/jdk) can run the following to never have to set `--from` on the command-line:

$ git config --global backport.from https://github.com/openjdk/jdk

Note that `git-backport` only requires a personal access token (PAT) in the case when the `--from` option contains a simple repository name, for example `jdk`. In all other cases there is no need for a PAT, making the command accessible for those who are not using PATs. 

Testing:
- [x] Manual testing on Linux x64

Thanks,
Erik

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

Commit messages:
 - cli: rework git-backport

Changes: https://git.openjdk.java.net/skara/pull/1025/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1025&range=00
  Stats: 187 lines in 5 files changed: 58 ins; 92 del; 37 mod
  Patch: https://git.openjdk.java.net/skara/pull/1025.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1025/head:pull/1025

PR: https://git.openjdk.java.net/skara/pull/1025