fix credential store filepath

[JENKINS-21016]
I encountered related issue with Atlassian Stash 2.10.0 repository.
```
fatal: could not read Username for 'https://myrepo': No such device or address
```

I think git cannot read credential temp file in.git/config.
So I fixed this problem and tested on below environments.

* CentOS 6.4, git 1.8.5.2 and Jenkins 1.544
* Mac OS X 10.9, git 1.8.3.4 (Apple Git-47) and Jenkins 1.544

Please review it.