Filter master branch correctly on initial mirror (#1747)

On the first synchronization, the clone has the implicit branch "master". This cannot be
changed in JGit. When we fetch the refs from the repository that should be mirrored, the
master branch of the clone will be updated to the revision of the remote repository (if
it has a master branch). If now the master branch shall be filtered from mirroring (ie.
if it is rejected), we normally would delete the ref in this clone. But because it is
the current branch, it cannot be deleted. We detect this and later, 
after we have pushed the result, delete the master branch by pushing 
an empty ref to the central repository.

scm-plugins/scm-git-plugin/src/test/java/sonia/scm/repository/spi/GitMirrorCommandTest.java

@@ -112,6 +112,36 @@ public class GitMirrorCommandTest extends AbstractGitCommandTestBase {
     }
   }
 
+  @Test
+  public void shouldFilterMasterBranchWhenFilteredOnInitialMirror() throws IOException, GitAPIException {
+    MirrorCommandResult result = callMirrorCommand(repositoryDirectory.getAbsolutePath(), c -> {
+      c.setFilter(new MirrorFilter() {
+        @Override
+        public Filter getFilter(FilterContext context) {
+          return new Filter() {
+            @Override
+            public Result acceptBranch(BranchUpdate branch) {
+              if (branch.getBranchName().equals("master")) {
+                return Result.reject("master not accepted");
+              } else {
+                return Result.accept();
+              }
+            }
+          };
+        }
+      });
+    });
+
+    assertThat(result.getResult()).isEqualTo(REJECTED_UPDATES);
+    assertThat(result.getLog())
+      .contains("- 000000000..fcd0ef183 master (master not accepted)");
+
+    try (Git createdMirror = Git.open(clone)) {
+      assertThat(createdMirror.branchList().call().stream().filter(r -> r.getName().contains("master")).findAny())
+        .isEmpty();
+    }
+  }
+
   @Test
   public void shouldCreateEmptyLogWhenNoChangesFound() {
     callMirrorCommand();