Enable Git config core.fsyncObjectFiles

Follow-up to investigation in https://gitlab.com/gitlab-org/gitlab-ce/issues/51680#note_180508774

It appears Git defaults to not fsync-ing certain object writes, even though there is a configuration option core.fsyncObjectFiles to do it anyway. To err on the side of caution we should probably turn on that configuration option everywhere we can.

set in omnibus's omnibus_gitconfig defaults
add git config command in source installation guide gitlab-ce/doc/install/installation.md
update default Git config in CNG

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information