Right now, it's only possible for admins to specify the Gitaly gitconfig
as a single key and value pair. While this works just fine in theory
when you only consider this in the context of the `[[git.config]]`
stanzas. We'll also migrate the global gitconfig from its current
location in `omnibus_gitconfig['system']` to a new key though, and it
would be great if we could use the same configuration format for both.
But we'd have to continue to do laborious string manipulation if we
wanted to use the key-value format for that new configuration as we
don't write `[[git.config]]` stanzas there, but gitconfig files.

Instead, introduce the ability to explicitly specify the Git config as a
touple of sections (`http`), subsections (`"http://example.com"`) and
keys (`insteadOf`) that together would form the complete key
`http."http://example.com".insteadOf`. Like this, we have sufficient
information to write both file formats without having to do any string
conversations at all.

In order to not break backwards compatibility with the just-introduced
`gitaly['gitconfig']`, we retain the ability to just specify a key.

Changelog: fixed