Add 'is:mergeable' operator to find changes without merge conflict

With this new search operator contributors can easily find changes
that need to be rebased.

Reviewers might find this new search operator useful to filter out
changes that are not mergeable.

This change introduces a new index schema version and reindexing the
changes is required.

Change-Id: Iba8447f643d44bc13721302e54d2059d79fee37f
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>
6 files changed