mdb looks like it stays locked

Looks like the lock.mdb stays open after exiting s3git

```
ls -la .s3git/mdb/
total 112
drwxr-xr-x  4 frankw  staff    136 Mar 21 11:05 .
drwxr-xr-x  4 frankw  staff    136 Mar 21 11:05 ..
-rw-r--r--  1 frankw  staff  49152 Mar 21 11:05 data.mdb
-rw-r--r--  1 frankw  staff   8192 Mar 21 11:05 lock.mdb
```
