git-fetch, git-branch: Support local --track via a special remote '.'

git-fetch, git-branch: Support local --track via a special remote '.'

This patch adds support for a dummy remote '.' to avoid having
to declare a fake remote like

        [remote "local"]
                url = .
                fetch = refs/heads/*:refs/heads/*

Such a builtin remote simplifies the operation of "git-fetch",
which will populate FETCH_HEAD but will not pretend that two
repositories are in use, will not create a thin pack, and will
not perform any useless remapping of names.  The speed
improvement is around 20%, and it should improve more if
"git-fetch" is converted to a builtin.

To this end, git-parse-remote is grown with a new kind of
remote, 'builtin'.  In git-fetch.sh, we treat the builtin remote
specially in that it needs no pack/store operations.  In fact,
doing git-fetch on a builtin remote will simply populate
FETCH_HEAD appropriately.

The patch also improves of the --track/--no-track support,
extending it so that branch.<name>.remote items referring '.'
can be created.  Finally, it fixes a typo in git-checkout.sh.

Signed-off-by: Paolo Bonzini  <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>