gitserver: remove stale commit-graph.lock

Created by: stefanhengl

We have seen that, occasionally, commit-graph.locks prevent a git repack from succeeding. Benchmarks on our dogfood cluster have shown that a commit-graph call for a 5GB bare repository takes less than 1 min. The lock is only held during a short period during this time. A 1-hour grace period is very conservative.

Test Plan:

(1) I ran "git commit-graph write" for the megarepo on the dogfood cluster to get a sense of a meaningful graceperiod.

(2) I ran a local instance of Sourcegraph and manually created a commit-graph.lock file.

cd .sourcegraph/repos/github.com/sourcegraph/sourcegraph/.git/objects/info
touch commit-graph.lock

I monitored the cleanup cycles and confirmed that the lock file stays in place.

I updated the lock file's timestamp.

touch -t <1 hour prior> commit-graph.lock

The lock file was removed.

(3) added a unit test