Dolt system tables should advertise most specific type on describe

When working on my Django example app I needed to use the `dolt_branches` table. I noticed it advertises it's text fields as `text` type.

```
mydatabase/main> describe dolt_branches;
+------------------------+----------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+------------------------+----------+------+-----+---------+-------+
| name | text | NO | PRI | NULL | |
| hash | text | NO | | NULL | |
| latest_committer | text | YES | | NULL | |
| latest_committer_email | text | YES | | NULL | |
| latest_commit_date | datetime | YES | | NULL | |
| latest_commit_message | text | YES | | NULL | |
| remote | text | YES | | NULL | |
| branch | text | YES | | NULL | |
+------------------------+----------+------+-----+---------+-------+
8 rows in set (0.00 sec)
```

Where possible, we should advertise the most specific type. For instance `hash` should be `char(20)` or whatever the limit is. `name` should be `varchar(200)` or whatever the actual limit on a the number of characters in a branch name is.