Synchronizing with an upstream template repository fails if the .template_version contains an incorrect tag.

[jenstroeger]

If that file is missing then

python-package-template/.github/workflows/sync-with-upstream.yaml

Lines 49 to 50 in 40e438b

	CURRENT_VERSION=$(test -f repo/.github/workflows/.template_version \
	&& cat repo/.github/workflows/.template_version || echo "v0.0.0")

sets CURRENT_VERSION to "v0.0.0" such that

python-package-template/.github/workflows/sync-with-upstream.yaml

Line 66 in 40e438b

git diff "$CURRENT_VERSION..$LATEST_VERSION" "$(find . docs/ .github/ .github/workflows/ .github/codeql/ -maxdepth 1 -type f ! -name """*.md""" ! -name """.template_version""")" > diff.patch

results in an error:

fatal: ambiguous argument 'v0.0.0..v2.8.0': unknown revision or path not in the working tree.

I think we should return a valid template tag here, or fail the workflow more gracefully. That error is also caused if the file exists and contains an invalid/non-existent tag.