auto-commit new video thumbnails generated in bin/cron/fetch_external_resources

[davidsbailey]

When videos are added to videos.csv, a cronjob on staging updates the database and the local filesystem to reflect the changes. An engineer must then scoop the images under dashboard/public/c/video_thumbnails/ or else videos and thumbnails will be mismatched. In the best case, a failure to do this leads to a failed DTT. In the worst case, if none of the mismatched thumbnails are covered by tests, a wrong video thumbnail could make it to production.

This PR modifies the cronjob to commit and push the new video thumbnails to origin/staging, minimizing the chances of a DTT which includes changes to videos.csv without the corresponding changes to dashboard/public/c/video_thumbnails/.

verification

initial conditions:

Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ git status
On branch auto-commit-video-thumbnails
Changes to be committed:
	modified:   bin/cron/fetch_external_resources

Untracked files:
	dashboard/public/c/video_thumbnails/218.jpg
Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ git difftool HEAD origin/auto-commit-video-thumbnails 
Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ 

cronjob run:

Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ bin/cron/fetch_external_resources 
[auto-commit-video-thumbnails d33ee3ea38] committing video thumbnails from bin/cron/fetch_external_resources in development
 1 file changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 dashboard/public/c/video_thumbnails/218.jpg
Counting objects: 6, done.
Compressing objects: 100% (5/5), done.
Writing objects: 100% (6/6), 564 bytes | 0 bytes/s, done.
Total 6 (delta 4), reused 0 (delta 0)
remote: Resolving deltas: 100% (4/4), completed with 4 local objects.
To github.com:code-dot-org/code-dot-org.git
   de193fd269..d33ee3ea38  auto-commit-video-thumbnails -> auto-commit-video-thumbnails

video thumbnails were committed and pushed:

Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ git log --stat -1
commit d33ee3ea38ce48682e7a378404f9c6c27f076140
Author: David Bailey <davidsbailey@users.noreply.github.com>
Date:   Fri Jul 7 17:32:18 2017 -0700

    committing video thumbnails from bin/cron/fetch_external_resources in development

 dashboard/public/c/video_thumbnails/218.jpg | 0
 1 file changed, 0 insertions(+), 0 deletions(-)
Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ git diff HEAD origin/auto-commit-video-thumbnails 
Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ 

non-video files in the git index were not committed:

Davids-MacBook-Pro-2:~/src/cdo (auto-commit-video-thumbnails)$ git status
On branch auto-commit-video-thumbnails
Changes to be committed:
	modified:   bin/cron/fetch_external_resources