Expose message prettification

This is useful outside of our command-emulating methods, so let a user
prettify as much as they want.

Author: carlosmn

LibGit2Sharp.Tests/CommitFixture.cs

@@ -1045,5 +1045,15 @@ public void CanNotAmendACommitInAWayThatWouldLeadTheNewCommitToBecomeEmpty()
                     new CommitOptions { AmendPreviousCommit = true }));
             }
         }
+
+        [Fact]
+        public void CanPrettifyAMessage()
+        {
+            string input = "# Comment\nA line that will remain\n# And another character\n\n\n";
+            string expected = "A line that will remain\n";
+
+            Assert.Equal(expected, Commit.PrettifyMessage(input, '#'));
+            Assert.Equal(expected, Commit.PrettifyMessage(input.Replace('#', ';'), ';'));
+        }
     }
 }

LibGit2Sharp/Commit.cs

@@ -113,6 +113,20 @@ private static string RetrieveEncodingOf(ObjectHandle obj)
             return encoding ?? "UTF-8";
         }
 
+        /// <summary>
+        /// Prettify a commit message
+        /// <para>
+        /// Remove comment lines and trailing lines
+        /// </para>
+        /// </summary>
+        /// <returns>The prettified message</returns>
+        /// <param name="message">The message to prettify.</param>
+        /// <param name="commentChar">Comment character. Lines starting with it will be removed</param>
+        public static string PrettifyMessage(string message, char commentChar)
+        {
+            return Proxy.git_message_prettify(message, commentChar);
+        }
+
         private string DebuggerDisplay
         {
             get