new: skip the git-init question inside existing repositories

src/new/git.rs

@@ -28,6 +28,23 @@ pub fn ensure_repository(dir: &Path, init: bool) -> Result<bool, Box<dyn std::er
     }
 }
 
+/// Whether `dir` sits inside a git work tree (its own or a parent's), in the
+/// spirit of `git rev-parse --is-inside-work-tree` (like
+/// [`crate::new::origin::GitOrigin::detect`]). Fail-soft: when `git` cannot
+/// be run or the probe fails, `dir` counts as outside — the caller keeps the
+/// behavior it would have without the probe, and the scaffold-time
+/// repository discovery in [`ensure_repository`] has the final word anyway.
+pub fn inside_work_tree(dir: &Path) -> bool {
+    let Ok(output) = std::process::Command::new("git")
+        .args(["rev-parse", "--is-inside-work-tree"])
+        .current_dir(dir)
+        .output()
+    else {
+        return false;
+    };
+    output.status.success() && String::from_utf8_lossy(&output.stdout).trim() == "true"
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -61,4 +78,22 @@ mod tests {
         assert!(!ensure_repository(&sub, true).unwrap());
         assert!(!sub.join(".git").exists());
     }
+
+    /// The probe answers like `git rev-parse --is-inside-work-tree`:
+    /// outside any repository it says no, inside one (at any depth, as a
+    /// skeleton mode scaffold would probe through its parent directory) it
+    /// says yes without touching the tree.
+    #[test]
+    fn inside_work_tree_follows_the_parent_repository() {
+        let dir = tempdir().unwrap();
+        assert!(!inside_work_tree(dir.path()));
+
+        git2::Repository::init(dir.path()).unwrap();
+        assert!(inside_work_tree(dir.path()));
+
+        let sub = dir.path().join("sub");
+        std::fs::create_dir(&sub).unwrap();
+        assert!(inside_work_tree(&sub));
+        assert!(!sub.join(".git").exists());
+    }
 }