Fix infinite rebuild loop with polling-based file watching

[Bizordec]

Problem

When watchfiles is using polling-based file watching, sphinx-autobuild enters an infinite rebuild loop. This happens because:

1. During a build, file creation inside directories updates their metadata (mtime)
2. watchfiles detects Change.modified events for these directories (metadata/mtime changes)
3. sphinx-autobuild triggers a rebuild
4. The rebuild modifies the directories again, creating a loop

Solution

Modify the watch_filter in RebuildServer.watch() to ignore Change.modified events for directories, while still accepting:

• Change.modified only for files (actual content changes)
• Change.created and Change.deleted for both files and directories (structure changes)