Introduce %<branch> as shortcut to the tracked branch
Often, it is quite interesting to inspect the branch tracked by a given
branch. This patch introduces a nice notation to get at the tracked
branch: '%<branch>' can be used to access that tracked branch.
A special shortcut '%' refers to the branch tracked by the current branch.
Suggested by Pasky.
This patch extends the function introduced to handle the nth-last branch
(via the {-<n>} notation); therefore that function name was renamed to
something more general.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>