Invocation with `FROM_SHA=0000000000000000000000000000000000000000` is broken

GitLab unconditionally sets FROM_SHA when invoking gitlab-elasticsearch-indexer, setting it to the empty SHA if necessary. Unfortunately, gitlab-elasticsearch-indexer doesn't handle this case, failing with this error message:

git@tester:~/git-data/repositories/root/foo.git$  ELASTIC_CONNECTION_INFO='{"url":["http://localhost:9200"]}' FROM_SHA=0000000000000000000000000000000000000000 TO_SHA=d677e041ea209db4d30f9fc0a7b4d358f3b18ad7 RAILS_ENV=development /opt/gitlab/embedded/bin/gitlab-elasticsearch-indexer 1 `pwd`
2017/04/19 14:56:34 Failed to open /var/opt/gitlab/git-data/repositories/root/foo.git: Bad from SHA (0000000000000000000000000000000000000000): object not found

We need to modify the integration tests so they test this scenario, and submit fixes.

mentioned in merge request !5 (merged)

closed via commit 84fca734

mentioned in commit 84fca734