Only include "already existing ..." comment in gitignore on conflict

This comment was previously confusing when there were no existing items
that conflict.

src/cargo/ops/cargo_new.rs

@@ -487,10 +487,14 @@ impl IgnoreList {
             _ => &self.ignore,
         };
 
-        let mut out = "\n\n#Added by cargo\n\
-                       #\n\
-                       #already existing elements are commented out\n\n"
-            .to_string();
+        let mut out = "\n\n#Added by cargo\n".to_string();
+        if ignore_items
+            .iter()
+            .any(|item| existing_items.contains(item))
+        {
+            out.push_str("#\n#already existing elements are commented out\n");
+        }
+        out.push('\n');
 
         for item in ignore_items {
             if existing_items.contains(item) {

tests/testsuite/init.rs

@@ -99,6 +99,33 @@ fn simple_git_ignore_exists() {
     cargo_process("build").cwd(&paths::root().join("foo")).run();
 }
 
+#[cargo_test]
+fn git_ignore_exists_no_conflicting_entries() {
+    // write a .gitignore file with one entry
+    fs::create_dir_all(paths::root().join("foo")).unwrap();
+    fs::write(paths::root().join("foo/.gitignore"), "**/some.file").unwrap();
+
+    cargo_process("init --lib foo --edition 2015")
+        .env("USER", "foo")
+        .run();
+
+    let fp = paths::root().join("foo/.gitignore");
+    let mut contents = String::new();
+    File::open(&fp)
+        .unwrap()
+        .read_to_string(&mut contents)
+        .unwrap();
+    assert_eq!(
+        contents,
+        "**/some.file\n\n\
+         #Added by cargo\n\
+         \n\
+         /target\n\
+         **/*.rs.bk\n\
+         Cargo.lock\n",
+    );
+}
+
 #[cargo_test]
 fn both_lib_and_bin() {
     cargo_process("init --lib --bin")