Improve arc land recovery from issues with the remote.
Summary:
Fixes T2138.
- When a pull fails, restore the original branch.
- When a push fails, complain about it really loudly.
NOTE: No test plan for push yet since I'm not sure this is the right remedy, see T2138 for discsusion.
Test Plan:
- Tested pull by changing "git pull" to "git xxpull" and running "arc land". Saw the pull fail and my original branch restored.
Reviewers: vrana, aran
Reviewed By: vrana
Maniphest Tasks: T2138
Differential Revision: https://secure.phabricator.com/D4265