Revert bogus merger optimizations

Revert "Fix checkout when preparing a ref"

This reverts commit 6eeb24743a4012613a679600247c70986ff1f63d.

Revert "Don't reset the local repo"

This reverts commit 96ee718c4b162d77ed58865a684b9bedd46c9cab.

Revert "Fetch specific refs on ref-updated events"

This reverts commit bfd5853957d53e231c0ccdf74004f726709aa07e.

Change-Id: I50ae4535e3189350d3cc3a7527f89d5cb8eec01d
Fix preprocessing for ref-updated events

The conditional that did a 'git remote update' for ref-updated
events (which otherwise don't affect the Merger) was wrong.

Change-Id: Icb2596df023279442613e10e13104a3621d867d9