MINDEXER-73 Incremental-only index update support

The patch provided by Steve Carlucci <scarlucci@sonatype.com>



git-svn-id: https://svn.apache.org/repos/asf/maven/indexer/trunk@1471039 13f79535-47bb-0310-9956-ffa450edef68
Merge branch 'trunk' into sonatype-release
[maven-release-plugin] prepare release maven-indexer-5.1.2-bf3d48b