Don't make len(p4Cmd("p4 changes -m 1 //foo/...")) == 0 succeed when the p4 command itself failed.

When the p4 command failed write out the exit code in the returned dict.

git-p4

@@ -31,7 +31,9 @@ def p4CmdList(cmd):
             result.append(entry)
     except EOFError:
         pass
-    pipe.close()
+    exitCode = pipe.close()
+    if exitCode != None:
+        result["p4ExitCode"] = exitCode
 
     return result