cvsimport: honor -i and non -i upon subsequent imports

cvsimport: honor -i and non -i upon subsequent imports

Documentation says -i is "import only", so without it,
subsequent import should update the current branch and working
tree files in a sensible way.

"A sensible way" defined by this commit is "act as if it is a
git pull from foreign repository which happens to be CVS not
git".  So:

 - If importing into the current branch (note that cvsimport
   requires the tracking branch is pristine -- you checked out
   the tracking branch but it is your responsibility not to make
   your own commits there), fast forward the branch head and
   match the index and working tree using two-way merge, just
   like "git pull" does.

 - If importing into a separate tracking branch, update that
   branch head, and merge it into your current branch, again,
   just like "git pull" does.

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