Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter assumes that we own everything beneath the base, as if it were being called with a base of the repository or working directory, and is tailored towards checkout and ensuring that there is no bogosity beneath the base that must be cleaned up. This is (at best) slow and (at worst) unsafe in the larger context of a filesystem where we do not own things and cannot do things like unlink symlinks that are in our way.
When the callback returns an error, we should stop immediately. This broke when trying to make sure we pass specific errors up the chain. This broke cancelling out of the loose backend's foreach.
This is a contract that we made in the library and that we need to uphold. The contents of a blob can never be NULL because several parts of the library (including the filter and attributes code) expect `git_blob_rawcontent` to always return a valid pointer.
git hardocodes these as objects which exist regardless of whether they are in the odb and uses them in the shell interface as a way of expressing the lack of a blob or tree for one side of e.g. a diff. In the library we use each language's natural way of declaring a lack of value which makes a workaround like this unnecessary. Since git uses it, it does however mean each shell application would need to perform this check themselves. This makes it common work across a range of applications and an issue with compatibility with git, which fits right into what the library aims to provide. Thus we introduce the hard-coded empty blob and tree in the odb frontend. These hard-coded objects are checked for before going to the backends, but after the cache check, which means the second time they're used, they will be treated as normal cached objects instead of creating new ones.
We assume that everything under GIT_DIR/objects/ is a directory. This is not necessarily the case if some process left a stray file in there. Check beforehand if we do have a directory and ignore the entry otherwise.
The git_odb_exists_prefix API was not dealing correctly when a later backend returned GIT_ENOTFOUND even if an earlier backend had found the object. Additionally, the unit tests were not properly exercising the API and had a couple mistakes in checking the results. Lastly, since the backends are not expected to behavior correctly unless all bytes of the short id are zero except for the prefix, this makes the ODB prefix APIs explicitly clear out the extra bytes so the user doesn't have to be as careful.