Add force_sync option to chromite.checkout.

Add force_sync option to chromite.checkout.

"repo sync" sometimes got stuck when it detect changes under it
going to sync.  Since this is done inside buildbot, we expect repo
sync to overwrite anything there.  Let me add option to force
sync.

BUG=657675

[Yoshisato Yanagisawa, 2016-10-20 01:34:18]

yyanagisawa@chromium.org changed reviewers:
+ dnj@chromium.org, iannucci@chromium.org

[iannucci, 2016-10-20 22:31:13]

On 2016/10/20 01:34:19, Yoshisato Yanagisawa wrote:

This seems fine to me, but dnj would be the best reviewer.

[dnj, 2016-10-22 15:13:38]

This can't land b/c there is no coverage of the new code. Recipe engine requires
100% coverage.

[Yoshisato Yanagisawa, 2016-10-24 02:35:26]

As I have written in
https://bugs.chromium.org/p/chromium/issues/detail?id=657675, I will withdraw
this cl.
This is not the issue we see every day.  We can tolerate to leave this as-is. 
Allowing it to break make it easy to investigate the cause.

[Yoshisato Yanagisawa, 2016-10-24 02:35:33]

Description was changed from

==========
Add force_sync option to chromite.checkout.

"repo sync" sometimes got stuck when it detect changes under it
going to sync.  Since this is done inside buildbot, we expect repo
sync to overwrite anything there.  Let me add option to force
sync.

BUG=657675
==========

to

==========
Add force_sync option to chromite.checkout.

"repo sync" sometimes got stuck when it detect changes under it
going to sync.  Since this is done inside buildbot, we expect repo
sync to overwrite anything there.  Let me add option to force
sync.

BUG=657675
==========