buck2: create a .buckroot in init

Summary:
Might as well. The user told us they want their project *here*, that seems
like a logical thing to do. Our current "look for the top-most .buckconfig"
approach has always been rather confusing.

Differential Revision: D44992850

fbshipit-source-id: c898e03a8360422fdb4a52ca28a449b38ba58284

app/buck2_client/src/commands/init.rs

@@ -219,7 +219,14 @@ fn set_up_gitignore(repo_root: &Path) -> anyhow::Result<()> {
     Ok(())
 }
 
+fn set_up_buckroot(repo_root: &Path) -> anyhow::Result<()> {
+    fs_util::write(repo_root.join(".buckroot"), "")?;
+    Ok(())
+}
+
 fn set_up_project(repo_root: &Path, git: bool, prelude: bool) -> anyhow::Result<()> {
+    set_up_buckroot(repo_root)?;
+
     if git {
         if !Command::new("git")
             .arg("init")