Micro-optimize MergeCgroupToDescriptors()

Introduce a local variable for an expression that occurs twice. Use
string append instead of string concatenation because the former is more
efficient.

Bug: 213617178
Test: Compile-tested only.
Change-Id: I6e8b9d63b10accb220216dc484dffd18f5c54ce7
Signed-off-by: Bart Van Assche <bvanassche@google.com>
diff --git a/libprocessgroup/setup/cgroup_map_write.cpp b/libprocessgroup/setup/cgroup_map_write.cpp
index 992cc2e..d5bce8f 100644
--- a/libprocessgroup/setup/cgroup_map_write.cpp
+++ b/libprocessgroup/setup/cgroup_map_write.cpp
@@ -147,12 +147,15 @@
 static void MergeCgroupToDescriptors(std::map<std::string, CgroupDescriptor>* descriptors,
                                      const Json::Value& cgroup, const std::string& name,
                                      const std::string& root_path, int cgroups_version) {
+    const std::string cgroup_path = cgroup["Path"].asString();
     std::string path;
 
     if (!root_path.empty()) {
-        path = root_path + "/" + cgroup["Path"].asString();
+        path = root_path;
+        path += "/";
+        path += cgroup_path;
     } else {
-        path = cgroup["Path"].asString();
+        path = cgroup_path;
     }
 
     uint32_t controller_flags = 0;