I'm thinking about adding more fields to this table, and the sha1_hash values
will make this tricker.
Therefore, remove the value, and adjust the existing code to cope. This commit
also adds a new test which coveres some of the changed functionality.
Add some new pages /branches and /branch/... as well as a new git_branches
table. Also extend the email processing to enter the branch information in to
the database.
Rather than just storing the URL in the guix_revisions and
load_new_guix_revision_jobs tables. This will help when storing more
information like tags and branches in the future.