Setting --shadow-file makes dmypy reload daemon(perf issue)

This is a bug report.

• Minimal repro

$ touch test.py
$ cp test.py /tmp/test.py
$ dmypy run test.py
Daemon started
Success: no issues found in 22 source files
$ dmypy run test.py
Success: no issues found in 1 source file
$ dmypy run -- --shadow-file test.py /tmp/test.py test.py 
Restarting: configuration changed
Daemon stopped
Daemon started
Success: no issues found in 22 source files

• What is the actual behavior/output?
  dmypy interprets setting --shadow-file as a "configuration change" and reloads the mypy daemon. This effectively counteracts the whole point of having a daemon since it takes a long time to reload the daemon.
• What is the behavior/output you expect?
  dmypy should not interpret --shadow-file changes as configuration changes. It should use the existing daemon instead of reloading.
• What are the versions of mypy and Python you are using?
  Python 3.8.2.
  mypy 0.790+dev.2e9afec83a5214f4a0eaf57a96034230b8460379
  The mypy install is from Github master as of thirty minutes ago.
• What are the mypy flags you are using? (For example --strict-optional)
  The repro above describes this. There are no mypy.ini (or friends) files in the directory.

Start by tracing how dmypy processes the --shadow-file option and decides that configuration has changed. Use the minimal reproduction to verify that changing the shadow file does not stop or restart the daemon, while type checking still succeeds with the expected source-file count.