Like visualising regex expressions, it's debatable whether any of the browser Git clients make the job of searching repos a trivial process.
But never fear, there's good docs on searching issues and searching code at Github. Or if you a looking for a fairly unique string somewhere in a repo, then Surch is your man. But there are helpers available as extensions- for example search for 'git' in ChromeStore. GitCheatSheet looks handy, but their website is gone.
And one can always sample a host of standalone Git clients, none of them appear to be particularly lightweight, however complexity of use is enough for some to beat a hasty retreat back to the more familiar browser interfaces.