cli: duplicate: add setting for templating the new commit descriptions

[winterqt]

This allows the customization of the duplicated commit descriptions.

An ideal use case for this is emulating git cherry-pick -x, as illustrated in the tests.

Checklist

If applicable:

• I have updated CHANGELOG.md
• I have updated the documentation (README.md, docs/, demos/)
• I have updated the config schema (cli/src/config-schema.json)
• I have added tests to cover my changes

[winterqt]

Could probably use a mention in the Git command table at the very least (if we decide to keep this as a flag and not a setting a la templates.draft_commit_description).

[winterqt]

Excluding the efficiency of writing 2x the commits with this option enabled, the main downside of this method is that the evolog looks ugly with the extra commit (IMO):

    ○  yostqsxw [email protected] 2001-02-03 08:05:15 4037fb05
    │  a
    ○  yostqsxw hidden [email protected] 2001-02-03 08:05:15 3d341b2f
    │  a
    ○  rlvkpnrz [email protected] 2001-02-03 08:05:09 a 2443ea76
    │  a
    ○  rlvkpnrz hidden [email protected] 2001-02-03 08:05:08 d8d5f980
       (empty) a

However, the op log looks good. :)

    @  44a208b90324 [email protected] 2001-02-03 04:05:15.000 +07:00 - 2001-02-03 04:05:15.000 +07:00
    │  duplicate 1 commit(s)
    │  args: jj duplicate a -T 'concat(description, "\n", "(cherry-picked from ", commit_id, ")")'
    │
    │  Changed commits:
    │  ○  + yostqsxw 4037fb05 a

Probably not that big of a deal, though; our working copy snapshots put more "wear" on the repo than this ever could.

[yuja]

The implementation generally looks good to me, thanks.

[yuja]

Thanks.

[winterqt]

Alright, this should be good to go now; would appreciate one more review pass. Thanks for the help!