archive: optionally add "virtual" files
With the `--add-virtual-file=<path>:<content>` option, `git archive` now
supports use cases where relatively trivial files need to be added that
do not exist on disk.
This will allow us to generate `.zip` files with generated content,
without having to add said content to the object database and without
having to write it out to disk.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
[jc: tweaked <path> handling]
Signed-off-by: Junio C Hamano <gitster@pobox.com>