git-p4: check for existence of repo dir before trying to create

When using git-p4 in this manner:

git-p4 clone //depot/path/project myproject

If "myproject" already exists as a dir, but not a valid git repo, it fails
to create the directory.

Signed-off-by: Kevin Green <Kevin.Green@morganstanley.com>

contrib/fast-import/git-p4

@@ -1400,7 +1400,8 @@ class P4Clone(P4Sync):
             self.cloneDestination = self.defaultDestination(args)
 
         print "Importing from %s into %s" % (', '.join(depotPaths), self.cloneDestination)
-        os.makedirs(self.cloneDestination)
+        if not os.path.exists(self.cloneDestination):
+            os.makedirs(self.cloneDestination)
         os.chdir(self.cloneDestination)
         system("git init")
         self.gitdir = os.getcwd() + "/.git"