major: Use git cli instead of grgit

[ajoberstar]

grgit is not really necessary for this plugin, decoupling it will improve a lot of things.

The expectation is for behavior equivalence, but I expect there will be some quirks from switching so this is considered a major version.

[ciscoo]

I think this change will help in resolving an issue I'm having internally at my company. Currently, publishing our documentation fails with an unhelpful error:

* What went wrong:
Execution failed for task ':documentation:gitPublishPush'.
  > org.ajoberstar.grgit.PushException: Failed to push: refs/heads/gh-pages to refs/heads/gh-pages (pre-receive hook declined)

JGit doesn't support several Git hooks and currently, any error raised is lost. Enabling debug logs for JGit also doesn't help either:

[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< unpack ok
[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< ng refs/heads/gh-pages pre-receive hook declined
[DEBUG] [org.eclipse.jgit.transport.PacketLineIn] git< 0000

We use owasp/sedated which is a pre-receive hook. It is a bit of a pain to figure out what is being blocked when the only log from JGit doesn't provide any information.

By using the git CLI directly, this should surface the error and anything else really which should help.

[ciscoo]

Some early feedback from trying out 5.0.0-beta.1, it does not seem to work with the lightweight checkout in Jenkins. Previously it did without issue.

Disabling that and manually cloning the repo yields a different error:

hint: Using 'master' as the name for the initial branch. This default branch name
hint: is subject to change. To configure the initial branch name to use in all
hint: of your new repositories, which will suppress this warning, call:
hint: 
hint: 	git config --global init.defaultBranch <name>
hint: 
hint: Names commonly chosen instead of 'master' are 'main', 'trunk' and
hint: 'development'. The just-created branch can be renamed via this command:
hint: 
hint: 	git branch -m <name>
fatal: couldn't find remote ref refs/heads/gh-pages

Doing a git branch -a before ./gradlew gitPublishPush yields:

+ git branch -a
* master
  remotes/origin/HEAD -> origin/master
  remotes/origin/gh-pages
  remotes/origin/master

So, to me there is some regression here I think since the previous implementation worked when using grgit/jgit.