Add start_point option for checkout command

[Vasfed]

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Description

This adds support for start-point git option when creating a new branch during checkout.
Fixes #249

[jcouball]

Hello @Vasfed! Thank you for this pull request.

I do have some requested changes in my review comments. In addition to those changes, can you also update the README.md to mention that this flag is supported?

Also, the build fails on Windows which you can see here:
https://github.com/ruby-git/ruby-git/actions/runs/3138166086/jobs/5154249814#step:4:41

===============================================================================
Error: test_checkout(TestLib):
  Git::GitExecuteError: git "--git-dir=D:/tmp/git_test1664642165171/working/.git" "--work-tree=D:/tmp/git_test1664642165171/working" "-c" "core.quotePath=true" "-c" "color.ui=false" checkout "-b" "test_checkout_b2" "master"  2>&1:error: Your local changes to the following files would be overwritten by checkout:
  	colon_numbers.txt
  	example.txt
  	scott/newfile
  	scott/text.txt
  Please commit your changes or stash them before you switch branches.
  Aborting
D:/a/ruby-git/ruby-git/lib/git/lib.rb:1144:in `command'
D:/a/ruby-git/ruby-git/lib/git/lib.rb:783:in `checkout'
D:/a/ruby-git/ruby-git/tests/units/test_lib.rb:86:in `test_checkout'
     83:
     84:   def test_checkout
     85:     assert(@lib.checkout('test_checkout_b',{:new_branch=>true}))
  => 86:     assert(@lib.checkout('test_checkout_b2', {new_branch: true, start_point: 'master'}))
     87:     assert(@lib.checkout('.'))
     88:     assert(@lib.checkout('master'))
     89:   end
===============================================================================

[jcouball]

Looks great! Thank you for the PR and the changes.

[Vasfed]

Fixed tests on windows

[jcouball]

I had missed the build errors during my last review but did notice them before merging.

Thank you for proactively fixing the problem.