Teach git-remote add to fetch and track

This adds three options to 'git-remote add'.

* -f (or --fetch) option tells it to also run the initial "git
fetch" using the newly created remote shorthand.

* -t (or --track) option tells it not to use the default
wildcard to track all branches.

* -m (or --master) option tells it to make the
remote/$name/HEAD point at a remote tracking branch other
than master.

For example, with this I can say:

$ git remote add -f -t master -t quick-start -m master \
jbf-um git://linux-nfs.org/~bfields/git.git/

to

(1) create remote.jbf-um.url;

(2) track master and quick-start branches (and no other); the
two -t options create these two lines:

fetch = +refs/heads/master:refs/remotes/jbf-um/master
fetch = +refs/heads/quick-start:refs/remotes/jbf-um/quick-start

(3) set up remotes/jbf-um/HEAD to point at jbf-um/master so
that later I can say "git log jbf-um"

Or I could do

$ git remote add -t 'ap/*' andy /home/andy/git.git

to make Andy's topic branches kept track of under refs/remotes/andy/ap/.

Other possible improvements I considered but haven't implemented
(hint, hint) are:

* reject wildcard letters other than a trailing '*' to the -t
parameter;

* make -m optional and when the first -t parameter does not
have the trailing '*' default to that value (so the above
example does not need to say "-m master");

* if -m is not given, and -t parameter ends with '*' (i.e. the
above defaulting did not tell us where to point HEAD at), and
if we did the fetch with -f, check if 'master' was fetched
and make HEAD point at it.

Signed-off-by: Junio C Hamano <junkio@cox.net>