Merge branch 'add-i-in-c-all-except-patch'

This patch series implements the rest of the commands in `git add -i`'s
main loop: `update`, `revert`, `add_untracked`, `patch`, `diff`, and
`quit`. Apart from `quit`, these commands are all very similar in that
they first build a list of files, display it, let the user choose which
ones to act on, and then perform the action.

Note that the `patch` command is not actually converted to C, not
completely at least: the built-in version simply hands off to `git
add--interactive` after letting the user select which files to act on.

The reason for this omission is practicality. Out of the 1,800+ lines of
`git-add--interactive.perl`, over a thousand deal *just* with the `git
add -p` part. I did convert that functionality already (to be
contributed in a separate patch series), discovering that there is so
little overlap between the `git add --patch` part and the rest of `git
add --interactive` that I could put the former into a totally different
file: `add-patch.c`. Just a teaser ;-)

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho