Replace the git library

[hughbris]

sebastian/git is deprecated and there are better alternatives that will allow us to simplify our code and the enrich the UI (eg. #10).

Mentioned in #4 too.

[pamtbaau]

The one mentioned in #4 (https://github.com/symplify/git-wrapper) requires PHP >= 8. Does that pose a limitation?

[hughbris]

Oh yeah it does for me ATM, unfortunately.

[hughbris]

I did another quick search. Only found ones I'd starred before. Lots of noise in these searches of course. Here are some findings.

Git.php: The docs are so basic and the open issues are so old that I am scared off.

git-php: feature-rich, simple, documented, maintained. worth following up.

Some more are listed in a similar GitSync issue but none suitable and it's reasonably obvious why in each case.

I like the idea of not requiring PHP exec permission in a library because it opens the plugin up to being used in more secure setups. The less environment requirements the better.

[hughbris]

Effectively completed in 941afbc and merged into develop in f52dbc0.