sha1_name.c: don't waste cycles in the @-parsing loop

The @-parsing loop unnecessarily checks for the sequence "@{" from
len - 2 unnecessarily.  We can safely check from len - 4: write out a
comment justifying this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
sha1_name.c: simplify @-parsing in get_sha1_basic()

Presently, the logic for @-parsing is horribly convoluted.  Prove that
it is very straightforward by laying out the three cases (@{N},
@{u[upstream], and @{-N}) and explaining how each case should be
handled in comments.  All tests pass, and no functional changes have
been introduced.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
remote.c: teach branch_get() to treat symrefs other than HEAD

Currently, branch_get() either accepts either a branch name, empty
string, or the magic four-letter word "HEAD".  Make it additionally
handle symbolic refs that point to a branch.

Update sha1_name.c:interpret_branch_name() to look for "@{", not '@'
(since '@' is a valid symbolic ref).

These two changes together make the failing test in t1508
(at-combinations) pass.  In other words, you can now do:

    $ git symbolic-ref @ HEAD

And expect the following to work:

    $ git rev-parse @@{u}

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
refs.c: make @ a pseudo-ref alias to HEAD

First, make sure that check_refname_format() rejects the a refname
beginning with a '@'.  Add a test to t1400 (update-ref) demonstrating
that update-ref forbids the user from updating a ref named "@".

Now, resolve_ref_unsafe() is built to resolve any refs that have a
corresponding file inside $GITDIR.  Our "@" ref is a special
pseudo-ref and does not have a filesystem counterpart.  So,
hard-interpret "@" as "HEAD" and resolve .git/HEAD as usual.  This
means that we can drop the 'git symbolic-ref @ HEAD' line in t1508
(at-combinations), and everything will continue working as usual.

If the user does manage to create a '.git/@' unsafely (via
symbolic-ref or otherwise), it will be ignored.

In practice, this means that you will now be able to do:

    $ git show @~1
    $ git log @^2

Advertise these features in the tests and documentation.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>