run-command: prettify -D_FORTIFY_SOURCE workaround

run-command: prettify -D_FORTIFY_SOURCE workaround

Current gcc + glibc with -D_FORTIFY_SOURCE try very aggressively to
protect against a programming style which uses write(...) without
checking the return value for errors.  Even the usual hint of casting
to (void) does not suppress the warning.

Sometimes when there is an output error, especially right before exit,
there really is nothing to be done.  The obvious solution, adopted in
v1.7.0.3~20^2 (run-command.c: fix build warnings on Ubuntu,
2010-01-30), is to save the return value to a dummy variable:

	ssize_t dummy;
	dummy = write(...);

But that (1) is ugly and (2) triggers -Wunused-but-set-variable
warnings with gcc-4.6 -Wall, so we are not much better off than when
we started.

Instead, use an "if" statement with an empty body to make the intent
clear.

	if (write(...))
		; /* yes, yes, there was an error. */

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>