ucm presents out-of-date current namespace to user

[aryairani]

Downloadable transcript:
https://gist.github.com/aryairani/7d6ab9b6d35b8f9ce45a9540c73ac148
This reproduction requires #4910.

It shouldn't say "old foo - old foo" at the end, but it does. However, if you were to close and reopen ucm, and display bar again, it would correctly say "new foo - new foo", I'm told.

Input:

## Simple update propagation

Updates that occur in one branch are propagated to the other. In this example, Alice updates `foo`, while Bob adds a new dependent `bar` of the original `foo`. When Bob's branch is merged into Alice's, her update to `foo` is propagated to his `bar`.

```ucm:hide
.> project.create-empty project
project/main> builtins.mergeio
```

Original branch:
```unison:hide
foo : Text
foo = "old foo"
```

```ucm:hide
project/main> add
project/main> branch alice
```

Alice's updates:
```unison:hide
foo : Text
foo = "new foo"
```

```ucm:hide
project/alice> update
project/main> branch bob
```

Bob's adds:
```unison:hide
bar : Text
bar = foo ++ " - " ++ foo
```
```ucm
project/bob> display bar
```

```ucm:hide
project/bob> add
```
Merge result:
```ucm
project/alice> merge /bob
project/alice> view foo bar
project/alice> display bar
```

```ucm:hide
.> project.delete project
```

Output:

## Simple update propagation

Updates that occur in one branch are propagated to the other. In this example, Alice updates `foo`, while Bob adds a new dependent `bar` of the original `foo`. When Bob's branch is merged into Alice's, her update to `foo` is propagated to his `bar`.

Original branch:
```unison
foo : Text
foo = "old foo"
```

Alice's updates:
```unison
foo : Text
foo = "new foo"
```

Bob's adds:
```unison
bar : Text
bar = foo ++ " - " ++ foo
```

```ucm
project/bob> display bar

  "old foo - old foo"

```
Merge result:
```ucm
project/alice> merge /bob

  I merged bob into alice.

project/alice> view foo bar

  bar : Text
  bar =
    use Text ++
    foo ++ " - " ++ foo
  
  foo : Text
  foo = "new foo"

project/alice> display bar

  "old foo - old foo"

```