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>

git-fetch.sh

@@ -157,7 +157,7 @@ then
 	fi
 fi
 
-fetch_native () {
+fetch_all_at_once () {
 
   eval=$(echo "$1" | git-fetch--tool parse-reflist "-")
   eval "$eval"
@@ -165,7 +165,9 @@ fetch_native () {
     ( : subshell because we muck with IFS
       IFS=" 	$LF"
       (
-	if test -f "$remote" ; then
+	if test "$remote" = . ; then
+	    git-show-ref $rref || echo failed "$remote"
+	elif test -f "$remote" ; then
 	    test -n "$shallow_depth" &&
 		die "shallow clone with bundle is not supported"
 	    git-bundle unbundle "$remote" $rref ||
@@ -188,7 +190,7 @@ fetch_native () {
 
 }
 
-fetch_dumb () {
+fetch_per_ref () {
   reflist="$1"
   refs=
   rref=
@@ -292,10 +294,10 @@ fetch_dumb () {
 fetch_main () {
 	case "$remote" in
 	http://* | https://* | ftp://* | rsync://* )
-		fetch_dumb "$@"
+		fetch_per_ref "$@"
 		;;
 	*)
-		fetch_native "$@"
+		fetch_all_at_once "$@"
 		;;
 	esac
 }