When we don't own a buffer (asize=0) we currently allow the usage of
grow to copy the memory into a buffer we do own. This muddles the
meaning of grow, and lets us be a bit cavalier with ownership semantics.

Don't allow this any more. Usage of grow should be restricted to buffers
which we know own their own memory. If unsure, we must not attempt to
modify it.

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).