[libFuzzer] Merge: print stats after reading the output corpus dir.

[Dor1s]

Summary

The purpose is to be able to extract the number of new edges added to
the original (i.e. output) corpus directory after doing the merge. Use case
example: in ClusterFuzz, we do merge after every fuzzing session, to avoid
uploading too many corpus files, and we also record coverage stats at that
point. Having a separate line indicating stats after reading the initial output
corpus directory would make the stats extraction easier for both humans and
parsing scripts.

Context: https://github.com/google/clusterfuzz/issues/802.

[Dor1s]

Use LOADED instead of INITED, plus fix alignment of keywords in the log

lib/fuzzer/FuzzerMerge.cpp

for (size_t i = M.FirstNotProcessedFile; i < M.Files.size(); i++) {
     TPC.CollectFeatures([&](size_t Feature) {
       if (AllFeatures.insert(Feature).second)
         UniqFeatures.insert(Feature);
     });
     TPC.UpdateObservedPCs();
     // Show stats.
     if (!(TotalNumberOfRuns & (TotalNumberOfRuns - 1)))
       PrintStats("pulse ");
+    if (TotalNumberOfRuns == M.NumFilesInFirstCorpus)
+      PrintStats("INITED ");
     // Write the post-run marker and the coverage.
     OF << "FT " << i;
     for (size_t F : UniqFeatures)
       OF << " " << F;
     OF << "\n";
     OF << "COV " << i;
     TPC.ForEachObservedPC([&](const TracePC::PCTableEntry *TE) {
       if (AllPCs.insert(TE).second)