Provide a convenience function that creates a buffer that can be provided
to callers but will not be freed via `git_buf_free`, so the buffer
creator maintains the allocation lifecycle of the buffer's contents.

Introduce `git_buf_grow_by` to incrementally increase the size of a
`git_buf`, performing an overflow calculation on the growth.

Decode base64-encoded text into a git_buf

Allows for inserting the same character n amount of times

There are a few places where we need to join three strings to
assemble a path.  This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).

This makes the git_buf struct that was used internally into an
externally available structure and eliminates the git_buffer.

As part of that, some of the special cases that arose with the
externally used git_buffer were blended into the git_buf, such as
being careful about git_buf objects that may have a NULL ptr and
allowing for bufs with a valid ptr and size but zero asize as a
way of referring to externally owned data.