Fix 'Argument list too long' error in :merge_licenses genrule

We pass every file in //third_party:srcs to merge_licenses.sh and when
trying to update protobuf that list grew too large and caused errors on
macOS.

With xargs we can split the list of files into smaller chunks.

Change-Id: I402813f14e35ca6dac393112ff4f3c963e789536
PiperOrigin-RevId: 154536807

src/main/java/com/google/devtools/build/lib/BUILD

@@ -1130,7 +1130,9 @@ genrule(
         "//third_party:srcs",
     ],
     outs = ["runtime/commands/LICENSE"],
-    cmd = "$(location merge_licenses.sh) $(SRCS) > \"$@\"",
+    # Use xargs to avoid the shell script complaining that is has been passed too many arguments.
+    # xargs calls the script multiple times with at most 1000 arguments per call.
+    cmd = "echo \"$(SRCS)\" | xargs -n1000 $(location merge_licenses.sh) > \"$@\"",
     tools = ["merge_licenses.sh"],
 )