git-add: introduce --edit (to edit the diff vs. the index)

git-add: introduce --edit (to edit the diff vs. the index)

With "git add -e [<files>]", Git will fire up an editor with the current
diff relative to the index (i.e. what you would get with "git diff
[<files>]").

Now you can edit the patch as much as you like, including adding/removing
lines, editing the text, whatever.  Make sure, though, that the first
character of the hunk lines is still a space, a plus or a minus.

After you closed the editor, Git will adjust the line counts of the
hunks if necessary, thanks to the --fixup-line-counts option of apply,
and commit the patch.  Except if you deleted everything, in which case
nothing happens (for obvious reasons).

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