Don't add extra space before RustDoc comments

In v0.11.0 the `sanitize_line` function was updated to add a space to
the beginning of non-empty RustDoc comments, but often those comments
already contain a space. This commit updates the check to ensure the
extra space is only added if it doesn't already exist.

Fixes #693.

prost-build/src/ast.rs

@@ -149,7 +149,7 @@ impl Comments {
 
         let mut s = RULE_URL.replace_all(line, r"<$0>").to_string();
         s = RULE_BRACKETS.replace_all(&s, r"\$1$2\$3").to_string();
-        if !s.is_empty() {
+        if !s.chars().next().map_or(false, |c| c == ' ') {
             s.insert(0, ' ');
         }
         s
@@ -202,6 +202,40 @@ pub struct Method {
 mod tests {
     use super::*;
 
+    #[test]
+    fn test_comment_append_with_indent_leaves_prespaced_lines() {
+        struct TestCases {
+            name: &'static str,
+            input: String,
+            expected: String,
+        }
+
+        let tests = vec![
+            TestCases {
+                name: "existing_space",
+                input: " A line with a single leading space.".to_string(),
+                expected: "/// A line with a single leading space.\n".to_string(),
+            },
+            TestCases {
+                name: "non_existing_space",
+                input: "A line without a single leading space.".to_string(),
+                expected: "/// A line without a single leading space.\n".to_string(),
+            },
+        ];
+        for t in tests {
+            let input = Comments {
+                leading_detached: vec![],
+                leading: vec![],
+                trailing: vec![t.input],
+            };
+
+            let mut actual = "".to_string();
+            input.append_with_indent(0, &mut actual);
+
+            assert_eq!(t.expected, actual, "failed {}", t.name);
+        }
+    }
+
     #[test]
     fn test_comment_append_with_indent_sanitizes_comment_doc_url() {
         struct TestCases {