Taking a page from core git's strbuf, this introduces git_buf_initbuf
which is an empty string that is used to initialize the git_buf ptr
value even for new buffers.  Now the git_buf ptr will always point to
a valid NUL-terminated string.

This change required jumping through a few hoops for git_buf_grow
and git_buf_free to distinguish between a actual allocated buffer
and the global initial value.  Also, this moves the allocation
related functions to be next to each other near the top of buffer.c.