show message when changelog command invoked with not existing tag

[usernam3]

When meta_commit changelog is called and passed tag does not exist it shows error :

meta_commit/lib/meta_commit/changelog/commands/commit_diff_examiner.rb:21:in `meta': undefined method `oid' for nil:NilClass (NoMethodError)

It would be better if meta_commit will check tag presence before command execution and notify the user about it.

Scenario that should pass when this issue is closed (add it to features/changelog_command.feature)

Scenario: Run command passing not existing tags
  Given three_commits_with_two_tags_and_changelog git repository
  When I run meta_commit command `changelog v0.0.1 v0.0.2`
  Then the output should contain exactly "#{message_that_tag_does_not_exist}"
  Then the changelog should be equal to fixture file "header"

[usernam3]

• Add new scenario to feature (features/changelog_command.feature)
• Run (rake features) and see it fails
• Handle the error (Maybe on cli.rb:35 level, skip and print message with error if commit is nil)
• See usage scenario is passing 💪