Handle packed refs with no trailing newline

I saw a repo in the wild today which had a master branch ref which was packed, but had no trailing newline. Git handled it fine, but libgit2 choked on it. Fix seems simple enough. If we don't see a newline, assume the end of the buffer is the end of the ref line.
Merge pull request #1193 from libgit2/no-newline-packed-refs

Handle packed refs with no trailing newline