While the `ENSURE_SIZE` macro gets a reference to both the buffer that
is to be resized and a new size, we were not consistently referencing
the passed buffer, but instead a variable `buf`, which is not passed in.
Funnily enough, we never noticed because our buffers seem to always be
named `buf` whenever the macro was being used.

Fix the macro by always using the passed-in buffer. While at it, add
braces around all mentions of passed-in variables as should be done with
macros to avoid subtle errors.

Found-by: Edward Thompson

The `ENSURE_SIZE` macro can be used to grow a buffer if its currently
allocated size does not suffice a required target size. While most of
the code already uses this macro, the `git_buf_join` and `git_buf_join3`
functions do not yet use it. Due to the macro first checking whether we
have to grow the buffer at all, this has the benefit of saving a
function call when it is not needed. While this is nice to have, it will
probably not matter at all performance-wise -- instead, this only serves
for consistency across the code.

Both the `git_buf_init` and `git_buf_attach` functions may call
`git_buf_grow` in case they were given an allocation length as
parameter. As such, it is possible for these functions to fail when we
run out of memory. While it won't probably be used anytime soon, it does
indeed make sense to also record this fact by returning an error code
from both functions. As they belong to the internal API only, this
change does not break our interface.

The `git_buf_init` function has an optional length parameter, which will
cause the buffer to be initialized and allocated in one step. This can
be used instead of static initialization with `GIT_BUF_INIT` followed by
a `git_buf_grow`. This patch does so for two functions where it is
applicable.