git-arc: vertically align tag values

[glebius]

Summary

Force vertical alignment for the two tags that git-arc adds.

[glebius]

The change is a matter of taste, of course. So I decided to add all contributors to the script to create a kind of a vote. Anybody else using git-arc are also very welcome to join the review and vote their (dis)agreement.

[jlduran]

It is fine by me, as long as it is also updated in:
https://cgit.freebsd.org/src/tree/tools/tools/git/hooks/prepare-commit-msg#n47
(and align all the trailers accordingly)

[jhb]

The tag we have been using in the commit logs back to the CVS days only has one tab after Reviewed by: and all our other tags like PR:, etc. are all aligned to the same width both here and in our commit log template, the format documented in the Committer's Guide, etc.. Note that only Differential Revision is further indented because its name is so long. We are probably going to change all the metadata lines to be hyphenated git-style trailers in the near future, and I don't think we should change this now given that.

E.g. see some a recent commit:

commit 6dafe8c1e46e58097b277ab9babb3ec5336713d7
Author: Muhammad Moinur Rahman <[e-mail]>
Date:   Tue Dec 10 18:24:43 2024 +0100

    Clean pkg cache in release media
    
    Recently wifi-firmware-kmod-release pkgs were added in the release
    installation media, but unfortunately the pkgs were not cleaned up after
    the installation. These kept the stale pkgs in the /var/cache. The pkgs
    should be removed with pkg clean.
    
    PR:             283112
    Reported by:    [e-mail]
    Reviewed by:    cperciva, emaste
    Approved by:    cperciva, emaste, re (implicit)
    MFC after:      3 days
    Sponsored by:   The FreeBSD Foundation
    Differential Revision:  https://reviews.freebsd.org/D47942

Your change would mis-align the Reviewed by: tag there unless we change all of them to add an extra tab.