Currently, there is only one caller that adds entries into an offset map, and
this caller first uses `git_offmap_put` to add a key and then set the value at
the returned index by using `git_offmap_set_value_at`. This is just too tighlty
coupled with implementation details of the map as it exposes the index of
inserted entries, which we really do not care about at all.

Introduce a new function `git_offmap_set`, which takes as parameters the map,
key and value and directly returns an error code. Convert the caller to make use
of it instead.