gitserver: set GIT_DIR in exec handler
Created by: keegancsmith
If a git disk gets in a bad state "dir" may no longer look like a git dir. This then leads to git trying to discover GIT_DIR and crawling all the way up the filesystem. This can lead to strange errors. This has been observed sometimes when we restore from backup.
Part of https://github.com/sourcegraph/sourcegraph/issues/8415#issuecomment-589179274