Directories are created prior to merge during clone, and prior to any
Git command run.
This directly addresses CVE-2017-11353.
When cloning a repo which includes data in a .ssh or .gnupg directory,
if those directories do not exist at the time of cloning, yadm will
create the directories with mask 0700 prior to merging the fetched data
into the work-tree.
When running a Git command and .ssh or .gnupg directories do not exist,
create those directories with mask 0700 prior to running the Git
command. However, do not create those directories if
yadm.auto-private-dirs is false.
