fetch_git shows an error if git is not installed

bin/ruby-build

@@ -116,8 +116,13 @@ fetch_git() {
   local git_ref="$3"
 
   echo "Cloning ${git_url}..." >&2
-  { git clone --depth 1 --branch "$git_ref" "$git_url" "${package_name}"
-  } >&4 2>&1
+
+  if type git 2>/dev/null; then
+    git clone --depth 1 --branch "$git_ref" "$git_url" "${package_name}" >&4 2>&1
+  else
+    echo "error: please install \`git\` and try again" >&2
+    exit 1
+  fi
 }
 
 build_package() {