During benchmarking, I see excessive use of vector.reserve().

Summary:
This code path can potentially accumulate multiple important_files for level 0.
But for other levels, it should have only one file in the
important_files, so it is ok not to reserve excessive space, is it not?

Test Plan: make check

Reviewers: haobo

Reviewed By: haobo

CC: reconnect.grayhat, leveldb

Differential Revision: https://reviews.facebook.net/D14349

db/version_set.cc

@@ -458,7 +458,9 @@ void Version::Get(const ReadOptions& options,
     // Get the list of files to search in this level
     FileMetaData* const* files = &files_[level][0];
     important_files.clear();
-    important_files.reserve(num_files);
+    if (level == 0) {
+      important_files.reserve(num_files);
+    }
 
     // Some files may overlap each other. We find
     // all files that overlap user_key and process them in order from