Extension cache spuriously has all files deleted

[milas]

Expected Behavior

• Tilt extensions are synced transparently without issue

Current Behavior

• Possible for all the files in the extension cache ($XDG_DATA_HOME/tilt-dev/tilt_modules) to be deleted during sync

Steps to Reproduce

No consistent repro - report came from a user using Tilt normally.

Context

The tilt_modules folder still had the Git metadata (.git/)` but all the source files were deleted. I imagine it tried to remove all files for a sync and got stuck in a weird state, where Git is up-to-date, but the repo working state is dirty.

The user also only has a single ExtensionRepo (the default tilt-dev/tilt-extensions).