"from" pin is dropped when saving query (regression at b08ceaf)

[philrz]

Repro is with Brim commit 9b2c9c4, though I've confirmed via binary search that this problem first showed up at commit b08ceaf, which is associated with the changes in #2472.

As shown in the attached video, when a "from" pin is present and the query is saved, the "from" pin is immediately dropped.

Repro.mp4

[philrz]

Verified in Brim commit 03322d0.

Repeating the repro steps as shown in the attached video, now the "from" pin remains intact when the query is saved.

Verify.mp4

Thanks @jameskerr!