Add support for auto-syncing.
Add support for auto-syncing.

Summary:
This is basically copied from Khan/arcanist:khan-bin/arc.  It uses a
simple timestamp-file to make sure we auto-update once a day.

Test Plan:
Ran
   git bigfile
before submitting.  Saw it fail with:
   Cannot pull with rebase: You have unstaged changes.
   Please commit or stash them.
   While updating the git-bigfiles binary, "git pull" failed in /home/csilvers/khan/devtools/git-bigfile/bin: Command '['git', 'pull', '-q']' returned non-zero exit status 1. Make sure the repo is clean.

Submitted this change and then tried again.  Saw it show the help
message.  Then saw /tmp/git-bigfile.pull exists.  Did it again and saw
the help file again.

Reviewers: chris

Reviewed By: chris

Differential Revision: https://phabricator.khanacademy.org/D17315

Khan/git-bigfile