for-each-ref: introduce %(HEAD) asterisk marker

'git branch' shows which branch you are currently on with an '*', but
'git for-each-ref' misses this feature.  So, extend its format with
%(HEAD) for the same effect.

Now you can use the following format in for-each-ref:

  %C(red)%(HEAD)%C(reset) %(refname:short)

to display a red asterisk next to the current ref.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
for-each-ref: introduce %(upstream:track[short])

Introduce %(upstream:track) to display "[ahead M, behind N]" and
%(upstream:trackshort) to display "=", ">", "<", or "<>"
appropriately (inspired by contrib/completion/git-prompt.sh).

Now you can use the following format in for-each-ref:

  %C(green)%(refname:short)%C(reset)%(upstream:trackshort)

to display refs with terse tracking information.

Note that :track and :trackshort only work with "upstream", and error
out when used with anything else.

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