github mirroring: fix case sensitivity

[hanwen]

we're confusing the case for username.

$ ls -l /zoekt/repos/github.com/Stackdriver/
total 48
drwxr-xr-x 7 root root 4096 Nov 5 2018 agent-packaging.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 cassandracloudbenchmark.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 jmxtrans-config-stackdriver.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 kubernetes-configs.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 metadata-agent.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-agent-service-configs.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-client-python.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-custommetrics-java.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-prometheus-e2e.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-prometheus.git
drwxr-xr-x 7 root root 4096 Nov 29 2018 stackdriver-prometheus-sidecar.git
drwxr-xr-x 7 root root 4096 Nov 5 2018 stackdriver-statsd-backend.git

..

Jul 31 15:25:25 zoekt-60gb-16cpu-localssd-drawfork-20180530 zoekt-indexserver[1768]: 2019/07/31 15:25:25 run [zoekt-mirror-github -dest /zoekt/repos -delete -user stackdriver]
Jul 31 15:25:27 zoekt-60gb-16cpu-localssd-drawfork-20180530 zoekt-indexserver[1768]: 2019/07/31 15:25:27 command [zoekt-mirror-github -dest /zoekt/repos -delete -user stackdriver] failed: exit status 1
Jul 31 15:25:27 zoekt-60gb-16cpu-localssd-drawfork-20180530 zoekt-indexserver[1768]: OUT:
Jul 31 15:25:27 zoekt-60gb-16cpu-localssd-drawfork-20180530 zoekt-indexserver[1768]: ERR: 2019/07/31 15:25:27 deleteStaleRepos: lstat /zoekt/repos/github.com/stackdriver: no such file or directory