contrib: add line offset information to file check function of check-code.py
This is a part of preparation to apply checking with check-code.py on
code fragments embedded in *.t test scripts.
This information will be useful to show correct line number in an
actual file for errors detected in code fragments embedded in *.t test
scripts.
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).