use shell built-in `command' instead of which(1)

it's specified by POSIX AFAIK and requires less redirections.

regress/sha

@@ -3,11 +3,11 @@
 # USAGE: ./sha in out
 # writes the sha256 of in to file out
 
-if which sha256 2>/dev/null >/dev/null; then
+if command -v sha256 >/dev/null; then
 	exec sha256 < "$1" > "$2"
 fi
 
-if which sha256sum 2>/dev/null >/dev/null; then
+if command -v sha256sum >/dev/null; then
 	sha256sum "$1" | awk '{print $1}' > "$2"
 	exit $?
 fi