Fix branch ref exist check

When a tag with the same name as the branch exists, the branch creation
process should work too. We should detect that the branch already
exists, and allow to force create it when the force option is used.

Bug: 582538
Change-Id: I3b350d03be8edcde10e97b2318343240ca896cb0

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java

diff --git a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java
index 87be813..7c1cbc3 100644
--- a/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java
+++ b/org.eclipse.jgit.test/tst/org/eclipse/jgit/api/BranchCommandTest.java
@@ -12,6 +12,7 @@
 import static org.junit.Assert.assertEquals;
 import static org.junit.Assert.assertFalse;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.fail;
 
 import java.util.List;
@@ -160,6 +161,20 @@ public void testCreateAndList() throws Exception {
 				- allBefore);
 	}
 
+  @Test
+  public void testExistingNameInBothBranchesAndTags() throws Exception {
+    git.branchCreate().setName("test").call();
+    git.tag().setName("test").call();
+
+    // existing name not allowed w/o force
+	assertThrows("Create branch with existing ref name should fail",
+			RefAlreadyExistsException.class,
+			() -> git.branchCreate().setName("test").call());
+
+    // existing name allowed with force option
+    git.branchCreate().setName("test").setForce(true).call();
+  }
+
 	@Test(expected = InvalidRefNameException.class)
 	public void testInvalidBranchHEAD() throws Exception {
 		git.branchCreate().setName("HEAD").call();

org.eclipse.jgit/src/org/eclipse/jgit/api/CreateBranchCommand.java

diff --git a/org.eclipse.jgit/src/org/eclipse/jgit/api/CreateBranchCommand.java b/org.eclipse.jgit/src/org/eclipse/jgit/api/CreateBranchCommand.java
index e1efb86..013e0ff 100644
--- a/org.eclipse.jgit/src/org/eclipse/jgit/api/CreateBranchCommand.java
+++ b/org.eclipse.jgit/src/org/eclipse/jgit/api/CreateBranchCommand.java
@@ -88,9 +88,7 @@ public Ref call() throws GitAPIException, RefAlreadyExistsException,
 		checkCallable();
 		processOptions();
 		try (RevWalk revWalk = new RevWalk(repo)) {
-			Ref refToCheck = repo.findRef(name);
-			boolean exists = refToCheck != null
-					&& refToCheck.getName().startsWith(R_HEADS);
+			boolean exists = repo.findRef(R_HEADS + name) != null;
 			if (!force && exists)
 				throw new RefAlreadyExistsException(MessageFormat.format(
 						JGitText.get().refAlreadyExists1, name));