Diff print callback that writes to a git_buf.

This function is provided not for you to call it directly, but instead so you can use it as a function pointer to the git_diff_print or git_patch_print APIs. When using those APIs, you specify a callback to actually handle the diff and/or patch data.

Use this callback to easily write that data to a git_buf buffer. You must pass a git_buf * value as the payload to the git_diff_print and/or git_patch_print function. The data will be appended to the buffer (after any existing content).