[MNG-7698] Allow comments in .mvn/maven.config (#1141)

Backport of https://github.com/apache/maven/pull/1134

---

https://issues.apache.org/jira/browse/MNG-7698

maven-embedder/src/main/java/org/apache/maven/cli/MavenCli.java

@@ -382,7 +382,8 @@ public class MavenCli {
 
             if (configFile.isFile()) {
                 try (Stream<String> lines = Files.lines(configFile.toPath(), Charset.defaultCharset())) {
-                    String[] args = lines.filter(arg -> !arg.isEmpty()).toArray(String[]::new);
+                    String[] args = lines.filter(arg -> !arg.isEmpty() && !arg.startsWith("#"))
+                            .toArray(String[]::new);
                     mavenConfig = cliManager.parse(args);
                     List<?> unrecognized = mavenConfig.getArgList();
                     if (!unrecognized.isEmpty()) {

maven-embedder/src/test/projects/mavenConfigProperties/.mvn/maven.config

@@ -1,3 +1,4 @@
+# a comment
 -T
 3
 -Drevision=1.3.0