internal: make lintdiff.py resilient against git errors (#3694)

## Why
So that it works on constrained checkouts. 

Codex complains "❌ make (fails: lintdiff cannot diff against missing
main revision)"

Author: denik

tools/lintdiff.py

@@ -16,7 +16,9 @@
 
 def parse_lines(cmd):
     # print("+ " + " ".join(cmd), file=sys.stderr, flush=True)
-    result = subprocess.run(cmd, stdout=subprocess.PIPE, encoding="utf-8", check=True)
+    result = subprocess.run(cmd, stdout=subprocess.PIPE, encoding="utf-8")
+    if result.returncode != 0:
+        return
     return [x.strip() for x in result.stdout.split("\n") if x.strip()]
 
 
@@ -36,29 +38,34 @@ def main():
         args.ref = "HEAD"
 
     # All paths are relative to repo root, so we need to ensure we're in the right directory.
-    gitroot = parse_lines(["git", "rev-parse", "--show-toplevel"])[0]
-    os.chdir(gitroot)
+    gitroot = parse_lines(["git", "rev-parse", "--show-toplevel"])
+    if gitroot:
+        os.chdir(gitroot[0])
 
     # Get list of changed files relative to repo root.
     # Note: Paths are always relative to repo root, even when running from subdirectories.
     # Example: Running from tools/ returns 'tools/lintdiff.py' rather than just 'lintdiff.py'.
     changed = parse_lines(["git", "diff", "--name-only", args.ref, "--", "."])
+    if changed is None:
+        cmd = ["golangci-lint"] + args.args
+    else:
+        # We need to pass packages to golangci-lint, not individual files.
+        # QQQ for lint we should also pass all dependent packages
+        dirs = set()
+        for filename in changed:
+            if "/testdata/" in filename:
+                continue
+            if filename.endswith(".go"):
+                d = os.path.dirname(filename)
+                dirs.add(d)
+
+        dirs = ["./" + d for d in sorted(dirs) if os.path.exists(d)]
+
+        if not dirs:
+            sys.exit(0)
+
+        cmd = ["golangci-lint"] + args.args + dirs
 
-    # We need to pass packages to golangci-lint, not individual files.
-    dirs = set()
-    for filename in changed:
-        if "/testdata/" in filename:
-            continue
-        if filename.endswith(".go"):
-            d = os.path.dirname(filename)
-            dirs.add(d)
-
-    dirs = ["./" + d for d in sorted(dirs) if os.path.exists(d)]
-
-    if not dirs:
-        sys.exit(0)
-
-    cmd = ["golangci-lint"] + args.args + dirs
     print("+ " + " ".join(cmd), file=sys.stderr, flush=True)
     os.execvp(cmd[0], cmd)