The `git_buf` type is now no longer a publicly available structure, and
the `git_buf` family of functions are no longer exported.

The deprecation layer adds a typedef for `git_buf` (as `git_userbuf`)
and macros that define `git_buf` functions as `git_userbuf` functions.
This provides API (but not ABI) compatibility with libgit2 1.0's buffer
functionality.

Within libgit2 itself, we take care to avoid including those deprecated
typedefs and macros, since we want to continue using the `git_buf` type
and functions unmodified.  Therefore, a `GIT_DEPRECATE_BUF` guard now
wraps the buffer deprecation layer.  libgit2 will define that.

We used to hard-code the octothorpe as the comment character and the
documentation still mentions this even though we accept the comment character as
a parameter.

Update the line to indicate this and clean up the first paragraph a bit.

This is implemented in trailer.c and borrows a large amount of logic
from Git core to ensure compatibility.

The comment char is configurable and we need to provide a way for the
user to specify which comment char they chose for their message.

A lot of the tests were checking for overflow, which we don't have
anymore, so we can remove them.