Assertion failure for L0-L1 compactions.

Summary:
For level-0 compactions, we try to find if can include more L0 files
in the same compaction run. This causes the 'smallest' and 'largest'
key to get extended to a larger range. But the suceeding call to
ParentRangeInCompaction() was still using the earlier
values of 'smallest' and 'largest',

Because of this bug, a file in L1 can be part of two concurrent
compactions: one L0-L1 compaction and the other L1-L2 compaction.

This should not cause any data loss, but will cause an assertion
failure with debug builds.

Test Plan: make check

Differential Revision: https://reviews.facebook.net/D10677

db/version_set.cc

@@ -2082,6 +2082,11 @@ Compaction* VersionSet::PickCompaction() {
     // which will include the picked file.
     c->inputs_[0].clear();
     current_->GetOverlappingInputs(0, &smallest, &largest, &c->inputs_[0]);
+
+    // If we include more L0 files in the same compaction run it can
+    // cause the 'smallest' and 'largest' key to get extended to a
+    // larger range. So, re-invoke GetRange to get the new key range
+    GetRange(c->inputs_[0], &smallest, &largest);
     if (ParentRangeInCompaction(&smallest, &largest,
                                 level, &c->parent_index_)) {
       delete c;