Make some syntax change to follow coding style.

This reverts commit 487f2a8287c4d1d41d39e05ac53d0e50c679d7e9, reversing
changes made to c6cf7f0e6927f1b264a7dda2467d70b9f744a01f.

When a branch is specified to check out in clone, update the remote
tracking `HEAD` to point to it.  This mimics git's behavior, when
`git clone -b <name>` is used.

Update `refs/remotes/origin/HEAD` as a symbolic link to the remote's
default branch.

We derive the branch name, even in our code, we shouldn't assume that
the branch will be "master".

When cloning an empty repository, we need to guess what the branch
structure should be; instead of hardcoding `master`, use the
`init.defaultBranch` setting it if it provided.

When compiling libgit2 with -DDEPRECATE_HARD, we add a preprocessor
definition `GIT_DEPRECATE_HARD` which causes the "git2/deprecated.h"
header to be empty. As a result, no function declarations are made
available to callers, but the implementations are still available to
link against. This has the problem that function declarations also
aren't visible to the implementations, meaning that the symbol's
visibility will not be set up correctly. As a result, the resulting
library may not expose those deprecated symbols at all on some platforms
and thus cause linking errors.

Fix the issue by conditionally compiling deprecated functions, only.
While it becomes impossible to link against such a library in case one
uses deprecated functions, distributors of libgit2 aren't expected to
pass -DDEPRECATE_HARD anyway. Instead, users of libgit2 should manually
define GIT_DEPRECATE_HARD to hide deprecated functions. Using "real"
hard deprecation still makes sense in the context of CI to test we don't
use deprecated symbols ourselves and in case a dependant uses libgit2 in
a vendored way and knows it won't ever use any of the deprecated symbols
anyway.