[Outreachy] Strategy to implement guix git log --pretty=<string>

[Magali]

Hello Guix,

As you might know, as part of my Outreachy internship I'm currently
working on implementing the subcommand 'guix git log', for browsing the
history of all packages. So far, it works with '--oneline' and
'--format=<FORMAT>', and FORMAT can be 'oneline', 'medium' or 'full'. If
you want to see it, the code can be found at
https://gitlab.com/magalilemes/guix

On the road to adding another option to the subcommand,
'--pretty=<string>' arose as an idea. With git log, you can do something
like
git log pretty=<string>
And this string can have placeholders, such as %h for showing the short
hash of a commit, and %s for showing the commit subject. For instance,
you could have git log --pretty="%h %s" and this would display the
commit history log with the short hash and subject of commits.

So, in order to implement 'guix git log --pretty=<string>', I'd like
help with a strategy to parse the string. Any examples, ideas and tips
would be really appreciated.

Cheers,

Magali