Implement value adding by exposing the underlying `set_multivar`,
which supports only string values in the underlying libgit2, so no
other typed overloads are provided at this point.

The counterpart for deleting keys exposes the underlying `delete_multivar`.
No regex-based overload is exposed for consistency with the existing `Set<T>`
overloads which don't expose it either.

Also exposed the boolean return value from the `Unset` calls which
is already present in the Proxy API.

Fixes libgit2#1719.

Implement value adding by exposing the underlying `set_multivar`,
which supports only string values in the underlying libgit2, so no
other typed overloads are provided at this point.

The counterpart for deleting keys exposes the underlying `delete_multivar`.
No regex-based overload is exposed for consistency with the existing `Set<T>`
overloads which don't expose it either.

Also exposed the boolean return value from the `Unset` calls which
is already present in the Proxy API.

Fixes libgit2#1719.

Implement value adding by exposing the underlying `set_multivar`,
which supports only string values in the underlying libgit2, so no
other typed overloads are provided at this point.

The counterpart for deleting keys exposes the underlying `delete_multivar`.
No regex-based overload is exposed for consistency with the existing `Set<T>`
overloads which don't expose it either.

Also exposed the boolean return value from the `Unset` calls which
is already present in the Proxy API.

Fixes libgit2#1719.

Fix