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.

If the user wants to keep a copy for themselves, they should make a
copy. It adds unnecessary complexity to make sure the returned entries
are valid until the builder is cleared.

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.

Rename git_tree_entry_byoid() to _byid() as per the convention.

This wasn't being tested and since it has a callback, I fixed it
even though the return value of this callback is not treated like
any of the other callbacks in the API.

When a tool needs to recreate the tree object (for example an
interface to another VCS), it needs to use the raw attributes,
forgoing any normalization.