Revert "Remove the dependency on git ls-files from cli/build.rs (#3568) (#3741)

# Description of Changes

This reverts commit 53f692d (#3568)

That PR broke the build if any of the directories used as templates are
not clean. It spammed lots of errors like:
```
error: `C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/client.pdb` wasn't a utf-8 file
 --> C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/client.pdb:0:49
  |

error: `C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/ref/client.dll` wasn't a utf-8 file
 --> C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/ref/client.dll:0:3
  |

error: `C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/refint/client.dll` wasn't a utf-8 file
 --> C:\Users\boppy\clockwork\SpacetimeDB\crates\cli/.templates/parent_parent_sdks_csharp_examples~_quickstart-chat_client/obj~/Debug/net8.0/refint/client.dll:0:3
```

# API and ABI breaking changes

None.

# Expected complexity level and risk

1

# Testing

- [x] `cargo build` passes on my repo now, which it wasn't before this
commit

Co-authored-by: Zeke Foppa <bfops@users.noreply.github.com>

Author: bfops

crates/cli/build.rs

@@ -34,6 +34,10 @@ fn nix_injected_commit_hash() -> Option<String> {
     }
 }
 
+fn is_nix_build() -> bool {
+    nix_injected_commit_hash().is_some()
+}
+
 fn find_git_hash() -> String {
     nix_injected_commit_hash().unwrap_or_else(|| {
         // When we're *not* building in Nix, we can assume that git metadata is still present in the filesystem,
@@ -156,10 +160,10 @@ fn generate_template_files() {
 }
 
 fn generate_template_entry(code: &mut String, template_path: &Path, source: &str, manifest_dir: &Path) {
-    let (template_files, resolved_base) = list_all_files(template_path, manifest_dir);
+    let (git_files, resolved_base) = get_git_tracked_files(template_path, manifest_dir);
 
-    if template_files.is_empty() {
-        panic!("Template '{}' has no files, check if the path is correct", source);
+    if git_files.is_empty() {
+        panic!("Template '{}' has no git-tracked files! Check that the directory exists and contains files tracked by git.", source);
     }
 
     // Example: /Users/user/SpacetimeDB
@@ -192,7 +196,7 @@ fn generate_template_entry(code: &mut String, template_path: &Path, source: &str
     code.push_str("    {\n");
     code.push_str("        let mut files = HashMap::new();\n");
 
-    for file_path in template_files {
+    for file_path in git_files {
         // Example file_path: modules/quickstart-chat/src/lib.rs (relative to repo root)
         // Example resolved_base: modules/quickstart-chat
         // Example relative_path: src/lib.rs
@@ -255,6 +259,18 @@ fn generate_template_entry(code: &mut String, template_path: &Path, source: &str
     code.push_str("    }\n\n");
 }
 
+/// Get a list of files tracked by git from a given directory
+fn get_git_tracked_files(path: &Path, manifest_dir: &Path) -> (Vec<PathBuf>, PathBuf) {
+    if is_nix_build() {
+        // When building in Nix, we already know that there are no untracked files in our source tree,
+        // so we just list all of the files.
+        list_all_files(path, manifest_dir)
+    } else {
+        // When building outside of Nix, we invoke `git` to list all the tracked files.
+        get_git_tracked_files_via_cli(path, manifest_dir)
+    }
+}
+
 fn list_all_files(path: &Path, manifest_dir: &Path) -> (Vec<PathBuf>, PathBuf) {
     let manifest_dir = manifest_dir.canonicalize().unwrap_or_else(|err| {
         panic!(
@@ -330,6 +346,37 @@ fn make_repo_root_relative(full_path: &Path, repo_root: &Path) -> PathBuf {
         })
 }
 
+fn get_git_tracked_files_via_cli(path: &Path, manifest_dir: &Path) -> (Vec<PathBuf>, PathBuf) {
+    let repo_root = get_repo_root();
+    let repo_root = repo_root.canonicalize().unwrap_or_else(|err| {
+        panic!(
+            "Failed to canonicalize repo_root path {}: {err:#?}",
+            repo_root.display(),
+        )
+    });
+
+    let resolved_path = make_repo_root_relative(&get_full_path_within_manifest_dir(path, manifest_dir), &repo_root);
+
+    let output = Command::new("git")
+        .args(["ls-files", resolved_path.to_str().unwrap()])
+        .current_dir(repo_root)
+        .output()
+        .expect("Failed to execute git ls-files");
+
+    if !output.status.success() {
+        return (Vec::new(), resolved_path);
+    }
+
+    let stdout = String::from_utf8(output.stdout).unwrap();
+    let files: Vec<PathBuf> = stdout
+        .lines()
+        .filter(|line| !line.is_empty())
+        .map(PathBuf::from)
+        .collect();
+
+    (files, resolved_path)
+}
+
 fn get_repo_root() -> PathBuf {
     let manifest_dir = get_manifest_dir();
     // Cargo doesn't expose a way to get the workspace root, AFAICT (pgoldman 2025-10-31).