Strictly match author/committer field and capture email address

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.