Fix calling git-p4 rebase from within a subdirectory (git rebase wants to be in toplevel)

Signed-off-by: Simon Hausmann <shausman@trolltech.com>

contrib/fast-import/git-p4

@@ -1197,6 +1197,8 @@ if cmd.needsGit:
         gitdir = ".git"
         if not isValidGitDir(gitdir):
             gitdir = mypopen("git rev-parse --git-dir").read()[:-1]
+            if os.path.exists(gitdir):
+                os.chdir(mypopen("git rev-parse --show-cdup").read()[:-1]);
 
     if not isValidGitDir(gitdir):
         if isValidGitDir(gitdir + "/.git"):