argv_array: offer to split a string by whitespace

argv_array: offer to split a string by whitespace

This is a simple function that will interpret a string as a whitespace
delimited list of values, and add those values into the array.

Note: this function does not (yet) offer to split by arbitrary delimiters,
or keep empty values in case of runs of whitespace, or de-quote Unix shell
style. All fo this functionality can be added later, when and if needed.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>