search: make sourcegraph.com index the repos with greatest updated_at - created_at
Created by: ijt
Fixes #4958
Also support the new environment variable $SOURCEGRAPH_REPOS_TO_INDEX_LIMIT
.
Test plan: Added automated tests and tested it locally by hard-coding the SourcegraphDotComMode
clause to true
and setting $SOURCEGRAPH_REPOS_TO_INDEX_LIMIT=1
.