closes #1

Correctly add new Commit objects to the commit map. put(...) returns the previously associated value leading to commits being overwritten.
Using computeIfAbsent(...) does not add Commits multiple times and prevents empty Optionals in the first invocation.

fix: empty commit was produced if there were no merge commits (String::split produced a 1 element array for an empty string)

remove contains check in Repository::checkout (problematic in conjunction with the copy method and the git call will fail anyway if the commit id is invalid), copy commits in Repository::copy