regression: --watch=foo only reloads on the first change to foo

[tjjfvi]

This is a regression between v1.29.2 and v1.29.3; likely caused by #15466.

To reproduce:

echo 'a' > file.txt
echo 'console.log(await Deno.readTextFile("file.txt"))' > cat.ts
deno run -A --watch=file.txt cat.ts &
echo 'b' >> file.txt
echo 'c' >> file.txt
echo 'd' >> file.txt

On v1.29.2, each echo causes the watcher to trigger; on v1.29.3 and v1.30.0, only the first echo does.