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

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

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

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

to display a red asterisk next to the current ref.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@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 the 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>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
for-each-ref: improve responsiveness of %(upstream:track)

Before anything is printed, for-each-ref sorts all refs first.  As
part of the sorting, populate_value() is called to fill the values in
for all atoms/placeholders per entry. By the time sort_refs() is done,
pretty much all data is already retrieved.

This works fine when data can be cheaply retrieved before
%(upstream:track) comes into the picture. It may take a noticeable
amount of time to process %(upstream:track) for each entry. All
entries add up and make --format='%(refname)%(upstream:track)' seem
hung for a few seconds, then display everything at once.

Improve the responsiveness by only processing the one atom (*) at a
time so that processing one atom for all entries (e.g. sorting) won't
cause much delay (unless you choose a "heavy" atom to process).

(*) This is not entirely correct. If you sort by an atom that needs
object database access, then it will fill all atoms that need odb.
Which is not a bad thing. We don't want to access odb once at sorting
phase and again at display phase.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>