Fix git fetch --unshallow on non-shallow clones

Only run git fetch --unshallow if .git/shallow exists, fixing the build
failure when the repository is already fully cloned (fetch-depth: 0).

Refs: #137 (comment)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: simonw

build.sh

@@ -2,7 +2,9 @@
 set -e
 
 # Make sure we have the full git history for finding commit dates:
-git fetch --unshallow
+if [ -f .git/shallow ]; then
+    git fetch --unshallow
+fi
 
 echo "=== Building tools.simonwillison.net ==="