Add "production" branch command

There already was a branch command for test purposes. This was adapted
for production use with git and hg.

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

@@ -0,0 +1,27 @@
+package sonia.scm.repository.spi;
+
+import org.assertj.core.api.Assertions;
+import org.junit.Test;
+import sonia.scm.repository.Branch;
+
+import java.io.IOException;
+import java.util.List;
+
+
+public class GitBranchCommandTest extends AbstractGitCommandTestBase {
+
+  @Test
+  public void shouldCreateBranch() throws IOException {
+    GitContext context = createContext();
+
+    Assertions.assertThat(readBranches(context)).filteredOn(b -> b.getName().equals("new_branch")).isEmpty();
+
+    new GitBranchCommand(context, repository).branch("new_branch");
+
+    Assertions.assertThat(readBranches(context)).filteredOn(b -> b.getName().equals("new_branch")).isNotEmpty();
+  }
+
+  private List<Branch> readBranches(GitContext context) throws IOException {
+    return new GitBranchesCommand(context, repository).getBranches();
+  }
+}