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.

Fix typo

Fix some type issues

More fixes

Address requested changes

Add test

Fix naming

Fix condition and tests

Address requested changes

Fix typo

Fixed a few header @param and @return typos with the help of -Wdocumentation in Xcode.

The following warnings have not been fixed:
common.h:213 - Not sure how the documentation format is for '...'
notes.h:102 - Correct @param name but empty text
notes.h:111 - Correct @param name but empty text
pack.h:140 - @return missing text
pack.h:148 - @return missing text

Bring back a way of acessing the git_refspec* from a remote.

Closes #1514

It used to be separate as an attempt to make the querying easier, but
it didn't work out that way, so put all the data together.

Add git_refspec_string() as well to get the original string, which is
now stored alongside the independent parts.