checkout proc startpoint (#3557)

* checkout proc startpoint

* zach comments

go/libraries/doltcore/sqle/dfunctions/dolt_checkout.go

@@ -78,6 +78,8 @@ func DoDoltCheckout(ctx *sql.Context, args []string) (int, error) {
 	if newBranch, newBranchOk := apr.GetValue(cli.CheckoutCoBranch); newBranchOk {
 		if len(newBranch) == 0 {
 			err = errors.New("error: cannot checkout empty string")
+		} else if len(apr.Args) > 0 {
+			err = checkoutNewBranch(ctx, dbName, dbData, roots, newBranch, apr.Arg(0))
 		} else {
 			err = checkoutNewBranch(ctx, dbName, dbData, roots, newBranch, "")
 		}

go/libraries/doltcore/sqle/enginetest/dolt_queries.go

@@ -1433,6 +1433,39 @@ var DoltBranchScripts = []queries.ScriptTest{
 			},
 		},
 	},
+	{
+		Name: "Create branch from startpoint",
+		SetUpScript: []string{
+			"create table a (x int)",
+			"set @commit1 = (select DOLT_COMMIT('-am', 'add table a'));",
+		},
+		Assertions: []queries.ScriptTestAssertion{
+			{
+				Query:    "show tables",
+				Expected: []sql.Row{{"a"}, {"myview"}},
+			},
+			{
+				Query:    "CALL DOLT_CHECKOUT('-b', 'newBranch', 'head~1')",
+				Expected: []sql.Row{{0}},
+			},
+			{
+				Query:    "show tables",
+				Expected: []sql.Row{{"myview"}},
+			},
+			{
+				Query:    "CALL DOLT_CHECKOUT('-b', 'newBranch2', @commit1)",
+				Expected: []sql.Row{{0}},
+			},
+			{
+				Query:    "show tables",
+				Expected: []sql.Row{{"a"}, {"myview"}},
+			},
+			{
+				Query:          "CALL DOLT_CHECKOUT('-b', 'otherBranch', 'unknownCommit')",
+				ExpectedErrStr: "fatal: 'unknownCommit' is not a commit and a branch 'otherBranch' cannot be created from it",
+			},
+		},
+	},
 }
 
 var DoltReset = []queries.ScriptTest{