docs: Remove non existing wrapper from the readme

libgit2net stopped to exist, all hail libgit2sharp
Merge pull request #2015 from txdv/patch-1

docs: Remove non existing wrapper from the readme
Accept 'submodule.*.fetchRecurseSubmodules' config 'on-demand' value
Default value for fetchRecurseSubmodules should be yes
Update test related to fetchRecurseSubmodules