Fixing behavior of --reload-dir

[galvana]

Summary

Changing the behavior of the --reload-dir flag in WatchFilesReload. Continuing off the discussion in #2107.

Current Behavior

Currently, when using the --reload flag with Uvicorn, the WatchFilesReload supervisor always watches the current working directory (cwd) in addition to any directories specified via --reload-dir. This means even when users explicitly specify which directories to watch, the supervisor still monitors the entire cwd.

New Behavior

This PR modifies the WatchFilesReload supervisor to:

• Only watch the directories explicitly specified via --reload-dir when provided
• Only watch the cwd when no directories are specified via --reload-dir
• Continue watching directories within cwd when they are explicitly specified

Motivation

This change makes the --reload-dir flag behavior more intuitive and efficient by:

• Preventing unnecessary file watching when specific directories are provided
• Reducing system resources used by the file watcher
• Giving users more precise control over which directories trigger reloads

Example

# Before: Watches both ./api and the entire cwd
uvicorn app:app --reload --reload-dir ./api

# After: Only watches ./api directory
uvicorn app:app --reload --reload-dir ./api

# After: Watches cwd since no directory specified
uvicorn app:app --reload

Checklist

• I understand that this PR may be closed in case there was no previous discussion. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.