lib/bash.rb: allow passing custom returns to safe_grep

Bash.safe_grep hardcodes option returns to [0,1], which prevents
callers from providing broader exit code acceptance (e.g. 2 for
missing files). Update to conditionally set options[:returns]
only if it is not already provided by the caller.

Signed-off-by: Philip Li <philip.li@intel.com>

Author: rli9

lib/bash.rb

@@ -70,7 +70,7 @@ def run(*args, **options, &block)
     end
 
     def safe_grep(*args, **options, &)
-      options[:returns] = [0, 1]
+      options[:returns] ||= [0, 1]
       run(*args, **options, &)
     end