Cannot exclude __init__.py without excluding the entire dir

Bug Report

To Reproduce

$ mkdir foo
$ cd foo
$ mkdir testpkg
$ echo "def f(): pass" > testpkg/__init__.py
$ echo "def g(): pass" > testpkg/foo.py

$  mypy --disallow-untyped-defs --exclude "fo" testpkg   # <--- excludes foo.py
testpkg/__init__.py:1: error: Function is missing a return type annotation
testpkg/__init__.py:1: note: Use "-> None" if function does not return a value
Found 1 error in 1 file (checked 1 source file)

$ mypy --disallow-untyped-defs --exclude "init" testpkg   # <---- should exclude __init__.py
testpkg/__init__.py:1: error: Function is missing a return type annotation
testpkg/__init__.py:1: note: Use "-> None" if function does not return a value
testpkg/foo.py:1: error: Function is missing a return type annotation
testpkg/foo.py:1: note: Use "-> None" if function does not return a value
Found 2 errors in 2 files (checked 1 source file)

Expected Behavior

__init__.py should be ignored in the last command

Actual Behavior

__init__.py was not ignored in the last command

Your Environment

• Mypy version used: 0.910
• Mypy command-line flags: see above
• Mypy configuration options from mypy.ini (and other config files): none
• Python version used: 3.7.3
• Operating system and version: Debian 10

Reproduce the two mypy commands using testpkg/init.py and testpkg/foo.py, comparing the --exclude "fo" and --exclude "init" results. Trace how the exclude pattern is applied to discovered files, then add coverage showing that __init__.py is ignored by the latter pattern while foo.py remains checked.