Allow a @ without branch in revision syntax

No branch before @ is interpreted as the currently checked out branch.
For detached heads it would be HEAD, but normally it is the branch
that HEAD refers to.

Change-Id: I051a1724fa390b8212e8986ba832b1347a20371e

org.eclipse.jgit.test/tst/org/eclipse/jgit/lib/ReflogResolveTest.java

@@ -71,6 +71,38 @@ public class ReflogResolveTest extends RepositoryTestCase {
 		assertEquals(c1, db.resolve("master@{1}"));
 	}
 
+	@Test
+	public void resolveUnnamedCurrentBranchCommits() throws Exception {
+		Git git = new Git(db);
+		writeTrashFile("file.txt", "content");
+		git.add().addFilepattern("file.txt").call();
+		RevCommit c1 = git.commit().setMessage("create file").call();
+		writeTrashFile("file.txt", "content2");
+		git.add().addFilepattern("file.txt").call();
+		RevCommit c2 = git.commit().setMessage("edit file").call();
+
+		assertEquals(c2, db.resolve("master@{0}"));
+		assertEquals(c1, db.resolve("master@{1}"));
+
+		git.checkout().setCreateBranch(true).setName("newbranch")
+				.setStartPoint(c1).call();
+
+		// same as current branch, e.g. master
+		assertEquals(c1, db.resolve("@{0}"));
+		try {
+			assertEquals(c1, db.resolve("@{1}"));
+			fail(); // Looking at wrong ref, e.g HEAD
+		} catch (RevisionSyntaxException e) {
+			assertNotNull(e);
+		}
+
+		// detached head, read HEAD reflog
+		git.checkout().setName(c2.getName()).call();
+		assertEquals(c2, db.resolve("@{0}"));
+		assertEquals(c1, db.resolve("@{1}"));
+		assertEquals(c2, db.resolve("@{2}"));
+	}
+
 	@Test
 	public void resolveReflogParent() throws Exception {
 		Git git = new Git(db);

org.eclipse.jgit/src/org/eclipse/jgit/lib/Repository.java

@@ -533,10 +533,22 @@ public abstract class Repository {
 				}
 				if (time != null) {
 					String refName = new String(revChars, 0, i);
-					Ref resolved = getRefDatabase().getRef(refName);
-					if (resolved == null)
+					Ref ref;
+					if (refName.equals("")) {
+						// Currently checked out branch, HEAD if
+						// detached
+						ref = getRef(Constants.HEAD);
+						if (ref == null)
+							return null;
+						if (ref.isSymbolic())
+							ref = ref.getLeaf();
+						if (ref.getObjectId() == null)
+							return null;
+					} else
+						ref = getRef(refName);
+					if (ref == null)
 						return null;
-					rev = resolveReflog(rw, resolved, time);
+					rev = resolveReflog(rw, ref, time);
 					i = m;
 				} else
 					i = m - 1;