Reindex: add --dry-run flag

We are working on evaluating performance of Lucene indexing, and want
to be able to separate the cost of Gerrit-specific operations from the
cost of Lucene operations. This flag prevents any writing to the
Lucene indexes, which will help with that. We may want to remove this
flag eventually.

Change-Id: I0e68f26c5d89c1c9b1b0b5d082af3b9241ce8d80
3 files changed