vsx: use 'git' and 'git-ui' builtin extensions

[vince-fugnitto]

Description

Now that we moved the git-history view into it's own extension #6381, can we update the examples present in the repo to use both git and git-ui builtins (while removing @theia/git)?

Moving towards the builtin support for Git will allow us to more easily test the extensions, and also allow us to identify compatibility issues more easily and quicker.

cc @westbury @thegecko

[thegecko]

Makes sense, I see no issue with that.

[akosyakov]

We are still using in Gitpod, hoped to switch from @theia/git after #6921.

But it should be alright to move Theia git extension support to own repo and test this repo against built-in extensions. cc @svenefftinge

@vince-fugnitto Could you add it to discuss at next dev meeting?

[akosyakov]

It depends on #7152