.gitignore file in cache is created too late

Bug Report

While mypy is running "infinitely long" (compare #12225), and it's run inside a Git repo, and it's run there for the first time, and the cache location is not set, ..., many cache files (>1900 in my case) are created before any .gitignore file appears. This confuses VS Code quite a bit.

I checked https://github.com/python/mypy/pull/8193/files, and either

1. this is related to mypy hanging during metastore creation (in which case it might make sense to create the .gitignore file immediately after creating the directory, and not after finishing the store), or
2. this is related to the cache directory being created in a different piece of code.

The fact that the gitignore file is created after I Ctrl-C the hanging mypy instance makes me think it's 1. So regardless of the root cause of #12225, I wonder if the .gitignore file should be created earlier.

Your Environment

• Mypy version used: 0.942
• Mypy command-line flags: none
• Mypy configuration options from mypy.ini (and other config files):

check_untyped_defs = true
disallow_incomplete_defs = true
disallow_untyped_calls = true
disallow_untyped_defs = true
show_error_codes = true

• Python version used: 3.10.3
• Operating system and version: Linux

Compare the cache and .gitignore creation flow with the changes shown in PR #8193, and consider the interaction with the metastore hang described in issue #12225. Reproduce a first run inside a Git repository with no configured cache location, then verify that .gitignore appears before large numbers of cache files are written and that the existing cache behavior remains intact.