Revert "Revert "include/git: don't re-download single-trees""

This reverts commit baa3d4f217.

include/git.sh

@@ -98,6 +98,10 @@ clone_project()
 
 	loc="${loc#src/}"
 	loc="src/${loc}"
+	if [ -d "${loc}" ]; then
+		printf "%s already exists, so skipping download\n" 1>&2
+		return 0
+	fi
 
 	git clone ${url} "${tmp_git_dir}" || \
 	    git clone ${bkup_url} "${tmp_git_dir}" || \