A data buffer for exporting data from libgit2

Sometimes libgit2 wants to return an allocated data buffer to the caller and have the caller take responsibility for freeing that memory. This can be awkward if the caller does not have easy access to the same allocation functions that libgit2 is using. In those cases, libgit2 will fill in a git_buf and the caller can use git_buf_dispose() to release it when they are done.

A git_buf may also be used for the caller to pass in a reference to a block of memory they hold. In this case, libgit2 will not resize or free the memory, but will read from it as needed.

Some APIs may occasionally do something slightly unusual with a buffer, such as setting ptr to a value that was passed in by the user. In those cases, the behavior will be clearly documented by the API.