Fixed ambiguous ls-remote with refs/heads/...

This is the smallest possible change to solve the
issue of CI builds being triggered again and again.
ls-remote will always be definite when "refs/heads/<branchName>"
is used as branchSpec.

All other issues tried to solve with Pull Request
#129
are still open. But using refs/heads/ would already solve
the CI issue.

src/main/java/org/jenkinsci/plugins/gitclient/CliGitAPIImpl.java

@@ -1857,8 +1857,8 @@ public Map<String, ObjectId> getHeadRev(String url) throws GitException, Interru
     }
 
     public ObjectId getHeadRev(String url, String branchSpec) throws GitException, InterruptedException {
-        final String branchName = normalizeBranchSpec(branchSpec);
-
+        final String branchName = (branchSpec.startsWith("refs/heads/")) 
+                    ? branchSpec :extractBranchNameFromBranchSpec(branchSpec);
         ArgumentListBuilder args = new ArgumentListBuilder("ls-remote");
         if(!branchName.startsWith("refs/tags/")) {
             args.add("-h");