fix(update-vscode): add check/docs for git-subtree (#3129)

ci/dev/update-vscode.sh

@@ -56,9 +56,21 @@ main() {
     echo "jq could not be found."
     echo "We use this when looking up the exact version to update to in the package.json in VS Code."
     echo -e "See docs here: https://stedolan.github.io/jq/download/"
-    exit
+    exit 1
   fi
 
+  # Note: `git subtree` returns 129 when installed, and prints help;
+  # but when uninstalled, returns 1.
+  set +e
+  git subtree &>/dev/null
+  if [ $? -ne 129 ]; then
+    echo "git-subtree could not be found."
+    echo "We use this to fetch and update the lib/vscode subtree."
+    echo -e "Please install git subtree."
+    exit 1
+  fi
+  set -e
+
   # Grab the exact version from package.json
   VSCODE_EXACT_VERSION=$(curl -s "https://raw.githubusercontent.com/microsoft/vscode/release/$VSCODE_VERSION_TO_UPDATE/package.json" | jq -r ".version")
 

docs/CONTRIBUTING.md

@@ -68,6 +68,9 @@ To develop inside an isolated Docker container:
 
 ### Updating VS Code
 
+Updating VS Code requires `git subtree`. On some rpm-based Linux distros, `git subtree` is not included by default, and needs to be installed separately.
+To install, run `dnf install git-subtree` or `yum install git-subtree` as necessary.
+
 To update VS Code, follow these steps:
 
 1. Run `yarn update:vscode`.