Start defining a more sophisticated run_command

Start defining a more sophisticated run_command

There are a number of places where we do some variation of
fork()+exec() but we also need to setup redirection in the process,
much like what run_command does for us already with its option flags.

It would be nice to reuse more of the run_command logic, especially
as that non-fork API helps us to port to odd platforms like Win32.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>