add possibility to reindex from scratch if cannot fast forward

[vladak]

Sometimes some repositories do rewrite history. This causes problem for automatic OpenGrok deployments (thinking these using opengrok-sync) as the project for such repository has to be reindexed from scratch. The tooling should be able to work around this, at least for select SCM types (Mercurial, Git).

[vladak]

This should be configurable on global and also per project level (in the opengrok-mirror configuration).