[RFC] Interactive difftool

Before git-difftool goes to master, i want to propose a new feature to
add to or replace the current behaviour of difftool. With current
difftool, we can only see the diff one by one. However, sometimes what
we want is to see the diff of selected files, or in a different order,
just like what we can do in the gui. So here is what i propose

$ git difftool --interactive [options]
[1] diff.c                   |   10 +++++++++-
[2] t/t4020-diff-external.sh |    8 ++++++++
Choose the file you want to see the diff of: 2

When the user types 2 and then <enter>, the external diff program is called

Further more, instead of just type a number, a letter can be prepended
to the number to represent different ways of diff. For example

t2 (tool 2): see the diff for file 2 with the configured diff tool
p2 (patch 2): see the diff for file 2 in the patch format

What do you think?

Ping Yin