conftest.py: set HGPLAIN=1 and GIT_PAGER=cat, remove now-obsolete --no-pager
testing: tidy up setting and asserting author fields
test_push_email: new xfail, email address is dropped on push
Strictly match author/committer field and capture email address (fixes test_push_email)

This regex attempts to precisely matches the specification given in
git-fast-import(1). With the old regex, the line

    "author Some Git <name@example.org> 1358978816 -0600"

produces

    user = "Some Git "

which ends up being pushed to Hg without the email. If the patch comes
back to git, it'll become "Some Git <unknown>", which causes
problems.