Fix initial multi-branch import.

The list of existing p4 branches in git wasn't initialized.

Signed-off-by: Simon Hausmann <shausman@trolltech.com>
diff --git a/contrib/fast-import/git-p4 b/contrib/fast-import/git-p4
index b3f27fe..e527734 100755
--- a/contrib/fast-import/git-p4
+++ b/contrib/fast-import/git-p4
@@ -637,6 +637,7 @@
         self.isWindows = (platform.system() == "Windows")
         self.keepRepoPath = False
         self.depotPaths = None
+        self.p4BranchesInGit = []
 
         if gitConfig("git-p4.syncFromOrigin") == "false":
             self.syncWithOrigin = False