re #611 - Ensures branch when using dynamic repositories with buildserver

When using a dynamic repository with a buildserver the current branch will
for most buildserver types return null or empty, this causes no branch to
be set, dynamic repositories require a branch.

The fix ensures that if we are using dynamic repositories with a
buildserver that if the buildserver does not return a branch we use the
targetBranch specified via /b

Author: Philo

src/GitVersionCore/ExecuteCore.cs

@@ -13,6 +13,8 @@ public static VersionVariables ExecuteGitVersion(IFileSystem fileSystem, string
             var applicableBuildServers = BuildServerList.GetApplicableBuildServers();
             var buildServer = applicableBuildServers.FirstOrDefault();
             var currentBranch = buildServer == null ? null : buildServer.GetCurrentBranch();
+
+            currentBranch = string.IsNullOrWhiteSpace(currentBranch) ? targetBranch : currentBranch;
             if (!string.IsNullOrEmpty(currentBranch))
             {
                 Logger.WriteInfo("Branch from build environment: " + currentBranch);