Added \b escape character as per spec (#1399)

Author: CEbbinghaus

src/Microsoft.Build.Tasks.Git.UnitTests/GitConfigTests.cs

@@ -350,6 +350,7 @@ public void ReadSectionHeader_Errors(string str, int unexpectedChar)
         [InlineData("name=\"a\rb\"", "name", "a\rb")]
         [InlineData("name=\"a\nb\"", "name", "a\nb")]
         [InlineData("name=\"a\r\nb\"", "name", "a\r\nb")]
+        [InlineData("name=\"a\bb\"", "name", "a\bb")]
         public void ReadVariableDeclaration(string str, string name, string value)
         {
             GitConfig.Reader.ReadVariableDeclaration(

src/Microsoft.Build.Tasks.Git/GitDataReader/GitConfig.Reader.cs

@@ -577,6 +577,14 @@ private static string ReadVariableValue(LineCountingReader reader, StringBuilder
                                 lengthIgnoringTrailingWhitespace = builder.Length;
                                 continue;
 
+                            case 'b':
+                                reader.Read();
+                                builder.Append('\b');
+
+                                // escaped \b is not considered trailing whitespace:
+                                lengthIgnoringTrailingWhitespace = builder.Length;
+                                continue;
+
                             case '\\':
                             case '"':
                                 builder.Append((char)reader.Read());