first scratch file save after edit is unresponsive

[ceedubs]

Describe and demonstrate the bug

• Edit a definition in your project: edit foo.
• Save your scratch file (with or without changes)
• ucm doesn't show any output (but if you have DEBUG=timing enabled, then it does show UnisonFileChanged)
• Save your scratch file a second time.
• Now it says Loading changes detected in... and works as expected

Screenshots

@cloud/nimbus/cluster-endpoints> edit main.nodeMain

  ☝️
  
  I added 1 definitions to the top of /home/unison/code/unison/scratch.u
  
  You can edit them there, then run `update` to replace the definitions currently in this
  namespace.

InputPattern: 141 ms (cpu), 144 ms (system)
@cloud/nimbus/cluster-endpoints> 
UnisonFileChanged: 7.03 µs (cpu), 9.03 µs (system)
@cloud/nimbus/cluster-endpoints> 

  Loading changes detected in ~/code/unison/scratch.u.


  I found and typechecked these definitions in ~/code/unison/scratch.u. If you do an `add` or
  `update`, here's how your codebase would change:
  
    ⍟ These names already exist. You can `update` them to your new definition:
    
      main.nodeMain : '{IO, Exception} ()

UnisonFileChanged: 473 ms (cpu), 475 ms (system)

Environment (please complete the following information):

• ucm --version a56abf7
• OS/Architecture: x86 NixOS

Additional context

I'm not sure, but I believe that this might be a regression introduced by #5631. cc @mitchellwrosen