Improved ignore pattern handling and flatten command for nested history

[pfn-asm]

• Apply the ignore patterns relative to the (nested) history they are in and not relative to the root history.
  • example: a pattern of /test.txt inside a nested history only applies to the one file at exactly this path inside the nested history, not a file test.txt inside the root history
• Transform an ignore patterns provided in a root history to be relative to the nested history where it applies.
  • example: an ignore pattern /A/AA/test.txt passed to the create command that applies to a file in a nested history (starting at /A/AA) is now stored as /test.txt only in that nested history.
• Further improve the ignore patterns handling to match the specification (handling of * wildcard, relative and absolute patterns, and handling of folders) and add many new tests.
• extend the flatten command to work with nested histories and the flattened manifest is not placed in a temporary folder anymore.
• Also Improve performance of the create command by removing unnecessary relative path resolution (relevant for histories > 1000 files)

[ptrpfn]

Went through the changes including new tests together with @pfn-asm in a review session – all looks good. Also closed other PRs in this session that are covered in this pull request.