fix reset colmap when counting distinct cols (#2096)

when using distinct cols with FindAndCount, reset statement.ColumnMap will make the counting sql an syntax error, this pr try fix this.
is this pr ok for merge?

error sql logging:
```sql
[SQL] SELECT DISTINCT `Fid` FROM `table_demo` WHERE Fkey = ? [val]

[SQL] SELECT count(DISTINCT ) FROM `table_demo` WHERE Fkey = ? [val]
```

after fix:
```sql
[SQL] SELECT DISTINCT `Fid` FROM `table_demo` WHERE Fkey = ? [val]

[SQL] SELECT count(DISTINCT `Fid`) FROM `table_demo` WHERE Fkey = ? [val]
```

Co-authored-by: finelog <kaicltw@gmail.com>
Reviewed-on: https://gitea.com/xorm/xorm/pulls/2096
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: finelog <finelog@noreply.gitea.io>
Co-committed-by: finelog <finelog@noreply.gitea.io>

integrations/session_find_test.go

@@ -711,6 +711,36 @@ func TestFindAndCountWithGroupBy(t *testing.T) {
 	assert.EqualValues(t, 2, len(results))
 }
 
+func TestFindAndCountWithDistinct(t *testing.T) {
+	assert.NoError(t, PrepareEngine())
+
+	type FindAndCountWithDistinct struct {
+		Id   int64
+		Age  int `xorm:"index"`
+		Name string
+	}
+
+	assert.NoError(t, testEngine.Sync(new(FindAndCountWithDistinct)))
+
+	_, err := testEngine.Insert([]FindAndCountWithDistinct{
+		{
+			Name: "test1",
+			Age:  10,
+		},
+		{
+			Name: "test2",
+			Age:  20,
+		},
+	})
+	assert.NoError(t, err)
+
+	var results []FindAndCountWithDistinct
+	cnt, err := testEngine.Distinct("`age`").FindAndCount(&results)
+	assert.NoError(t, err)
+	assert.EqualValues(t, 2, cnt)
+	assert.EqualValues(t, 2, len(results))
+}
+
 type FindMapDevice struct {
 	Deviceid string `xorm:"pk"`
 	Status   int

session_find.go

@@ -57,7 +57,7 @@ func (session *Session) FindAndCount(rowsSlicePtr interface{}, condiBean ...inte
 	if session.statement.SelectStr != "" {
 		session.statement.SelectStr = ""
 	}
-	if len(session.statement.ColumnMap) > 0 {
+	if len(session.statement.ColumnMap) > 0 && !session.statement.IsDistinct {
 		session.statement.ColumnMap = []string{}
 	}
 	if session.statement.OrderStr != "" {