index: merge of index fails

I've tried to index the Go sources on Windows. The first run of Cindex is like expected. The second run doesn't remove `.csearchindex~` and has created a second file `.csearchindex~~`. I don't know whether the merge of `.csearchindex` and `.csearchindex~` has been successful.

The expected behaviour is `.csearchindex` and `.csearchindex~` will be merged and `.csearchindex~` will removed.

OS: Windows 7
Go: go1.5rc1
### First Run

```
v% cindex go
2015/08/06 09:36:34 index E:\home\visfj\go
2015/08/06 09:36:34 flush index
2015/08/06 09:36:34 merge 0 files + mem
2015/08/06 09:36:35 39563770 data bytes, 9212960 index bytes
2015/08/06 09:36:35 done
v% ls -al
total 9068
drwxr-xr-x 1 visfj Domänen-Benutzer 0 Aug 6 09:36 .
drwxr-xr-x 1 visfj Domänen-Benutzer 0 Jul 30 13:22 ..
-rw-r--r-- 1 visfj Domänen-Benutzer 6213 Aug 6 08:36 .bash_history
-rw-r--r-- 1 visfj Domänen-Benutzer 422 Aug 5 11:29 .bashrc
-rw-r--r-- 1 visfj Domänen-Benutzer 9212960 Aug 6 09:36 .csearchindex
-rw-r--r-- 1 visfj Domänen-Benutzer 105 Aug 5 14:26 .gitconfig
-rw-r--r-- 1 visfj Domänen-Benutzer 114 Jul 31 13:56 .gitcookies
```
### Second Run

```
v% cindex
2015/08/06 09:36:45 index E:\home\visfj\go
2015/08/06 09:36:46 flush index
2015/08/06 09:36:46 merge 0 files + mem
2015/08/06 09:36:46 39563770 data bytes, 9212960 index bytes
2015/08/06 09:36:46 merge E:\home\visfj\.csearchindex E:\home\visfj\.csearchindex~
2015/08/06 09:36:46 done
v% ls -al
total 27068
drwxr-xr-x 1 visfj Domänen-Benutzer 0 Aug 6 09:36 .
drwxr-xr-x 1 visfj Domänen-Benutzer 0 Jul 30 13:22 ..
-rw-r--r-- 1 visfj Domänen-Benutzer 6213 Aug 6 08:36 .bash_history
-rw-r--r-- 1 visfj Domänen-Benutzer 422 Aug 5 11:29 .bashrc
-rw-r--r-- 1 visfj Domänen-Benutzer 9212960 Aug 6 09:36 .csearchindex
-rw-r--r-- 1 visfj Domänen-Benutzer 9212960 Aug 6 09:36 .csearchindex~
-rw-r--r-- 1 visfj Domänen-Benutzer 9212944 Aug 6 09:36 .csearchindex~~
-rw-r--r-- 1 visfj Domänen-Benutzer 105 Aug 5 14:26 .gitconfig
-rw-r--r-- 1 visfj Domänen-Benutzer 114 Jul 31 13:56 .gitcookies
```