CheckoutCommand: force flag now allows overwrite

CheckoutCommand: force flag now allows overwrite

Before this commit, a force checkout would fail if there
were any conflicting files. After this commit, a force
checkout will overwrite the conflicting files, as expected.

Making this work required fixing a bug in DirCacheCheckout.
Before this commit, when DirCacheCheckout had
failOnConflict=false, it would delete all conflicting files
from the working copy and just leave them missing. After
this commit, DirCacheCheckout overwrites conflicting files
with the merge tree.

This change in DirCacheCheckout causes "reset --hard" and
"revert --abort" to behave as expected (previously they
would simply delete conflicting files, now they will be
overwritten from the merge tree).

Change-Id: If7e328ee792ef6511ab7d9c26d8d77c39210ec9f
Signed-off-by: Ned Twigg <ned.twigg@diffplug.com>