replace all string::find_last_of() with string::rfind()

find_last_of was working properly only because it was searching
only for one character, string::rfind() is appropriate function
for this job.

src/helpers.cpp

@@ -364,7 +364,7 @@ string FindSharedDir(const string &one, const string &two)
 	while (one.substr(0, i) == two.substr(0, i))
 		i++;
 	result = one.substr(0, i);
-	i = result.find_last_of("/");
+	i = result.rfind("/");
 	return i != string::npos ? result.substr(0, i) : "/";
 }