I wonder if we should hoist this check up above the isDirty check.  There's a weird edge case here where I'm not sure what the right answer is:
1. make some unsaved edits
2. modify file externally
3. in Brackets, choose to keep your local edits
4. undo so the file is in a clean state
5. switch to another app and then back to Brackets

On master (and also with this PR, I think), this will cause the editor to silently switch to showing the external changes.  That feels surprising, since I already told Brackets to ignore those changes.

OTOH, if we continued to respect the choice made in the dialog you'd just get the same surprise later: Brackets lets you close the file without asking you to save it, since the doc is clean.  But then you go look on disk and it disagreed with what was in Brackets.

It's almost as if, while `keepChangesTime` is non-null, we should force the dirty flag to always be on.  That's a bigger change though, so I'm happy to spin off a separate bug -- just curious what you think the right behavior even is...