refs.c: interpret @ as HEAD

The rev spec forms @{}, .., ... fill in HEAD as the missing argument
automatically.  Unfortunately, HEAD~<n> is a very common idiom and
there is no way to make HEAD implicit here (due the shell expansion of
~<n>).

However, there is an alternative solution to the issue: overload the
character @ to mean HEAD.  Do this at the lowest possible layer of
abstraction: in dwim_ref(), substitute @ with HEAD just before calling
resolve_ref_unsafe().  The program will only reach this point after
the other specs like ~, ^ and @{} have been resolved; therefore, it is
safe to do it here.

This patch has the exact same effect as:

    $ git symbolic-ref @ HEAD

It means that you can now do @~1, @^2, and even topic..@.  However,
since the @-parsing happens before we ever reach the symref
resolution, @@{u} is invalid.  But this is okay, since @{u} already
has an implicit HEAD in it.

Inspired-by: Felipe Contreras <felipe.contreras@gmail.com>
Inspired-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>