This function is a constructor, so let's name it like one and leave _create() for the reference functions, which do create/write the reference.
Path validation may be influenced by `core.protectHFS` and `core.protectNTFS` configuration settings, thus treebuilders can take a repository to influence their configuration.
There are some combination of objects and target types which we know cannot be fulfilled. Return EINVALIDSPEC for those to signify that there is a mismatch in the user-provided data and what the object model is capable of satisfying. If we start at a tag and in the course of peeling find out that we cannot reach a particular type, we return EPEEL.
The old `allocfmt` is of no use to callers, as they are not able to free the returned buffer. Export a new API that returns a static string that doesn't need to be freed.
Finding a filename in a vector means we need to resort it every time we want to read from it, which includes every time we want to write to it as well, as we want to find duplicate keys. A hash-map fits what we want to do much more accurately, as we do not care about sorting, but just the particular filename. We still keep removed entries around, as the interface let you assume they were going to be around until the treebuilder is cleared or freed, but in this case that involves an append to a vector in the filter case, which can now fail. The only time we care about sorting is when we write out the tree, so let's make that the only time we do any sorting.