pathutil: add the more efficient finddir iterator (to be used in subsequent commits)

typing: import unconditionally This is needed if we're writing python3 signatures.

tests: check how hgweb handles HEAD requests This test file is loosely based on test-hgweb.t. HEAD support originally implemented in fda5a4b853ab.

tests: test hg status --all with hgext/git There's a TODO item in git/dirstate.py about obtaining clean files in a more straightforward way, let's first test that status can and does show clean files at all.