Add start_point option for checkout command

Co-authored-by: V.Kolesnikov <re.vkolesnikov@gmail.com>
Signed-off-by: Vasily Fedoseyev <vasilyfedoseyev@gmail.com>

README.md

@@ -265,6 +265,7 @@ g.branch('existing_branch').checkout
 g.branch('master').contains?('existing_branch')
 
 g.checkout('new_branch')
+g.checkout('new_branch', new_branch: true, start_point: 'master')
 g.checkout(g.branch('new_branch'))
 
 g.branch(name).merge(branch2)

lib/git/lib.rb

@@ -764,11 +764,21 @@ def branch_delete(branch)
       command('branch', '-D', branch)
     end
 
+    # Runs checkout command to checkout or create branch
+    #
+    # accepts options:
+    #  :new_branch
+    #  :force
+    #  :start_point
+    #
+    # @param [String] branch
+    # @param [Hash] opts
     def checkout(branch, opts = {})
       arr_opts = []
       arr_opts << '-b' if opts[:new_branch] || opts[:b]
       arr_opts << '--force' if opts[:force] || opts[:f]
       arr_opts << branch
+      arr_opts << opts[:start_point] if opts[:start_point] && arr_opts.include?('-b')
 
       command('checkout', arr_opts)
     end

tests/units/test_lib.rb

@@ -87,6 +87,19 @@ def test_checkout
     assert(@lib.checkout('master'))
   end
 
+  def test_checkout_with_start_point
+    assert(@lib.reset(nil, hard: true)) # to get around worktree status on windows
+
+    actual_cmd = nil
+    @lib.define_singleton_method(:run_command) do |git_cmd, &block|
+      actual_cmd = git_cmd
+      super(git_cmd, &block)
+    end
+
+    assert(@lib.checkout('test_checkout_b2', {new_branch: true, start_point: 'master'}))
+    assert_match(%r/checkout ['"]-b['"] ['"]test_checkout_b2['"] ['"]master['"]/, actual_cmd)
+  end
+
   # takes parameters, returns array of appropriate commit objects
   # :count
   # :since