Our current configuration logic is completely oblivious of any
repository, but only cares for actual file paths. Unfortunately, we are
forced to break this assumption by the introduction of conditional
includes, which are evaluated in the context of a repository. Right now,
only one conditional exists with "gitdir:" -- it will only include the
configuration if the current repository's git directory matches the
value passed to "gitdir:".

To support these conditionals, we have to break our API and make the
repository available when opening a configuration file. This commit
extends the `open` call of configuration backends to include another
repository and adjusts existing code to have it available. This includes
the user-visible functions `git_config_add_file_ondisk` and
`git_config_add_backend`.

This special-casing ignores that we might have a locked file, so the
hashtable does not represent the contents of the file we want to
write. This causes multivar writes to overwrite entries instead of add
to them when under lock.

There is no need for this as the normal code-path will write to the file
just fine, so simply get rid of it.

When we delete an entry, we also want to refresh the configuration to
catch any changes that happened externally.

This allows us to simplify the logic, as we no longer need to delete
these variables internally. The whole state will be refreshed and the
deleted entries won't be there.