Fix RevertCommand to correctly revert multiple commits at once.

The fix is to move the new head commit to the newly-created revert
commit, so that additional revert commits will use the correct head.

Change-Id: I5de3a9a2a4c276e60af732e9c507cbbdfd1a4652
Signed-off-by: Maik Schreiber <blizzy@blizzy.de>