tests: rely on PublicInbox::Git for pathname safety

tests: rely on PublicInbox::Git for pathname safety

It's possible (but unlikely) a user will put spaces in TMPDIR
and cause File::Temp::tempdir() to return a temporary directory
with spaces in the filename, making it unsafe for shell
expansion.

PublicInbox::Git didn't exist when t/mda.t was written, and
I just forgot about PublicInbox::Git->qx for t/plack.t :x