Avoid a method missing error distracting when we hit any unhandled exceptions

[brrygrdn]

I overzealously pruned methods from UpdateFilesCommand which resulted in unhandled errors hitting a problem where base_commit_sha wasn't defined making for some distracting traces, e.g.

 Dependabot encountered '1' error(s) during execution, please check the logs for more details.
updater | +---------------+
updater | |    Errors     |
updater | +---------------+
updater | | unknown_error |
updater | +---------------+
updater | /home/dependabot/dependabot-updater/lib/dependabot/base_command.rb:42:in `base_commit_sha': NotImplementedError (NotImplementedError)
updater | 	from /home/dependabot/dependabot-updater/lib/dependabot/base_command.rb:56:in `rescue in run'
updater | 	from /home/dependabot/dependabot-updater/lib/dependabot/base_command.rb:54:in `run'
updater | 	from bin/update_files.rb:23:in `<main>'
updater | /home/dependabot/dependabot-updater/lib/dependabot/updater/group_dependency_file_batch.rb:40:in `block in merge': undefined method `path' for nil:NilClass (NoMethodError)
updater | 
updater |                            Dependabot.logger.debug("Updated an unexpected file at '#{existing_file.path}'")
updater |                                                                                                   ^^^^^
...