git-gui: Remove unnecessary /dev/null redirection.

Git 1.5.0 and later no longer output useless messages to standard
error when making the initial (or what looks to be) commit of a
repository.  Since /dev/null does not exist on Windows in the
MinGW environment we can't redirect there anyway.  Since Git
does not output anymore, I'm removing the redirection.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>