Updated Git, Coreutils and Emacs, new script to prepare tarballs

Until now, one had to follow the instructions from [1] to prepare a
standard software tarball before merging with the low-level
tarballs-software repository [2]. The script only worked for '.tar.gz'
suffix and was only available as a comment on Savannah (in [1]).

With this commit, the script has been imported into Maneage as
'reproduce/software/shell/tarball-prepare.sh' to simplify future software
updates. It work with all supported '.tar.*' suffixes (of the upstream
tarball repository) and will convert the tarballs to Maneage's standard
format. Also, this script has a minimal argument parser and can skip the
tarballs that are already unpacked, allowing faster tests.

This script was used to update the versions of:

    Coreutiles    9.0  --> 9.1
    Git           2.34 --> 2.36
    Emacs         27.2 --> 28.1

The main motive behind this update was Git which announced a vulnerability
issue [3] and suggested an update to the latest version as soon as
possible. More detail is described in this github blog [4], but in summary,
it was a security issue on multi-user systems that has been found and fixed
by Git developers. Since Maneage is often installed on such shared systems,
it was important to make this update. GNU Coreutils and GNU Emacs were also
updated because they are also commonly used.

The following improvements have also done with this commit:

 - .gitignore: ignore emacs auto-save files (that end with a '#')

 - README-hacking.md: In the checklist for updating the Maneage branch, the
   no-longer-necessary '--decorate' option of Git was removed from the
   command to check the general branch history.

[1] https://savannah.nongnu.org/task/?15699
[2] https://git.maneage.org/tarballs-software.git/
[3] https://lore.kernel.org/git/xmqqv8veb5i6.fsf@gitster.g/
[4] https://github.blog/2022-04-12-git-security-vulnerability-announced/

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