Adds a srcCore (multi-valued) parameter through which one or more source core names can be given.
We use the IW.addIndexes(IndexReader...) method to merge the source core's indexes to the target core's index. Even if an IW is open on the source indexes, using a reader protects against corruption.
Note - although the indexDir param also ends up calling the IW.addIndexes(IndexReader...) method, we cannot protect against open IWs on the directory so the caveat of calling commit before using mergeindexes with indexDir param still applies.
A commit needs to be called after a merge action to see the changes.