Since we now always build the library with cdecl calling conventions, our callbacks should be decorated as such so that users will not be able to provide callbacks defined with other calling conventions. The `GIT_CALLBACK` macro will inject the `__cdecl` attribute as appropriate.
git_object_t is the future; update the public API to use it. This will also ensure that we can build our tests which make use of the old API without modification (and without compiler warnings).
The documentation for `git_treebuilder_insert` erroneously states that we do not validate that the entry being inserted exists. We do, as of https://github.com/libgit2/libgit2/pull/3633. Update the documentation to reflect the new reality.
The function to write trees allocates a new buffer for each tree. This causes problems with performance when performing a lot of actions involving writing trees, e.g. when doing many merges. Fix the issue by instead handing in a shared buffer, which is then re-used across the calls without having to re-allocate between calls.
Instead of going through the usual steps of reading a tree recursively into an index, modifying it and writing it back out as a tree, introduce a function to perform simple updates more efficiently. `git_tree_create_updated` avoids reading trees which are not modified and supports upsert and delete operations. It is not as versatile as modifying the index, but it makes some common operations much more efficient.
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.