A crash on parsing was fixed a while ago. This adds support for fully
correctly parsing multiline git config values, using git config --null.
Since git-annex-shell configlist uses normal git config output, I left in
support for that too; the two forms of config output can be easily
identified by the parser. Since configlist only prints the annex.uuid
config, there's no risk of multiline values there, so no need to change it.
