Reorganize the builtin driver table slightly so that core Git
builtin definitions can be imported verbatim. Then take a few of
the core Git drivers and pull them in.
This also creates a test of diffs with the builtin HTML driver
which led to some small error handling fixes in the driver
selection logic.
It's hard or even impossible to correctly free the string buffer
allocated by git_patch_to_str in some circumstances. Drop the function
so people have to use git_patch_to_buf instead - git_buf has a dedicated
destructor.
