Fix single-branch imports by skipping the branch/merge detection correctly.

Signed-off-by: Simon Hausmann <hausmann@kde.org>

contrib/fast-import/p4-fast-export.py

@@ -210,7 +210,7 @@ def commit(details, files, branch, branchPrefix):
     merges = Set()
 
     for file in files:
-        if lastChange == 0:
+        if lastChange == 0 or not detectBranches:
             continue
         path = file["path"]
         if not path.startswith(branchPrefix):