Normalize GetSnapshot archive permissions
In order to better ensure gitaly operations are not modifying repository data in read-only transactions, the read snapshot of all the involved repositories has its permissions set to readonly. This means that RPCs that are intended to generate an archive end up returning a very permission limited archive. So here we change the tar builder used for backing up custom hooks to generate an archive with read/write permissions. These permissions will be restricted by umask on extraction. Similar to how os.Create uses 0o666.
Edited by James Fargher