Improve comment rewriting with normalize_comments == false

Only change multiline comments of the form

```rust
/*
 * Text
 */
```

while not affecting comments of the form

```rust
/*
Text
*/
```

when normalize_comments is off. In the first case,
we have a known character we can align against, while
we don't have one in the second case.

Before, we have converted the second form into the first,
but this is against the spirit of normalize_comments being
turned off.

Fixes #956

src/comment.rs

@@ -71,6 +71,16 @@ pub fn rewrite_comment(orig: &str,
     let indent_str = offset.to_string(config);
     let line_breaks = s.chars().filter(|&c| c == '\n').count();
 
+    let num_bare_lines = s.lines()
+        .enumerate()
+        .map(|(_, line)| line.trim())
+        .filter(|l| !(l.starts_with('*') || l.starts_with("//") || l.starts_with("/*")))
+        .count();
+
+    if num_bare_lines > 0 && !config.normalize_comments {
+        return Some(orig.to_owned());
+    }
+
     let lines = s.lines()
         .enumerate()
         .map(|(i, mut line)| {

tests/source/comment4.rs

@@ -33,3 +33,15 @@ fn test() {
   /// test123
 fn doc_comment() {
 }
+
+/*
+Regression test for issue #956
+
+(some very important text)
+*/
+
+/*
+fn debug_function() {
+    println!("hello");
+}
+// */

tests/target/comment4.rs

@@ -32,3 +32,15 @@ fn test() {
 
 /// test123
 fn doc_comment() {}
+
+/*
+Regression test for issue #956
+
+(some very important text)
+*/
+
+/*
+fn debug_function() {
+    println!("hello");
+}
+// */