cmdutil: make walkchangerevs() gracefully handle wdir parents This code will be completely rewritten, but test-grep.t would fail without fixing it.

grep: fix --follow with no --diff nor --rev to not fall back to plain grep Before, "grep --follow" would only print matches in the working directory. Since --follow is the option to specify the search space, it should disable the plain (i.e. wdir) grep.

test-grep: add tests for --follow with/without --diff and/or paths This tests the behavior of cmdutil.walkchangerevs(). I'm going to rewrite cmdutil.walkchangerevs() to leverage the "log -f" logic, but the code coverage looked quite small. And the history traversal of "grep -f" goes wrong in various ways.

grep: clarify that --all works exactly the same way as --diff 7fbb5d76c555 "grep: add --diff flag" says as such, but the help strings have diverged since, and it's getting fuzzier what this --all will do.

churn: add an usage example in docstring

tests: run test-check-py3-compat only in pure python mode This test was added few years ago to detect syntax and import related warnings and kickstart the python-3 porting efforts. It tries to parse the ast and import the module. When using in strict modes like `HGMODULEPOLICY=rust` or `HGMODULEPOLICY=c`, the test fails. Let's run the test only on pure python mode. We now have full test suite being run with python 3 and some people even use hg with python 3 in production (like me).