check-manifest should parse and consider .gitignore file

[jhermann]

"missing from VCS" lists files I already excluded via .gitignore; needing to list them a 2nd time in the tool config is waste and more brittle that it'd need to be.

https://github.com/mherrmann/gitignore_parser is a maintained implementation.

check-manifest version 0.45

cf. #60

[mgedmin]

This is a very good idea!