Fix testOnlyReadsPartialInput on Darwin.

--
MOS_MIGRATED_REVID=86431291
diff --git a/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java b/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
index 4531cd5..694972e 100644
--- a/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
+++ b/src/test/java/com/google/devtools/build/lib/shell/CommandTest.java
@@ -456,7 +456,8 @@
 
   @Test
   public void testOnlyReadsPartialInput() throws Exception {
-    Command command = new Command(new String[] {"head", "--bytes", "500"});
+    // -c == --bytes, but -c also works on Darwin.
+    Command command = new Command(new String[] {"head", "-c", "500"});
     OutputStream out = new ByteArrayOutputStream();
     InputStream in = new InputStream() {